From 294a0572c47128564078ab78eb84bce5bdb80a79 Mon Sep 17 00:00:00 2001
From: Tim Newsome <tim@sifive.com>
Date: Mon, 13 Feb 2017 21:29:26 -0800
Subject: [PATCH] Implement program buffer preexec/postexec.

I only tested preexec.
---
 riscv/debug_defines.h | 779 ++++++++++++++++++++++++++++++++++++++++--
 riscv/debug_module.cc |  56 ++-
 riscv/debug_module.h  |  11 +-
 3 files changed, 802 insertions(+), 44 deletions(-)

diff --git a/riscv/debug_defines.h b/riscv/debug_defines.h
index 5666f46..59fa344 100644
--- a/riscv/debug_defines.h
+++ b/riscv/debug_defines.h
@@ -1,16 +1,45 @@
 #define AC_ACCESS_REGISTER                  None
+/*
+* 2: Access the lowest 32 bits of the register.
+*
+* 3: Access the lowest 64 bits of the register.
+*
+* 4: Access the lowest 128 bits of the register.
+*
+* If \Fsize specifies a size larger than the register is, then the
+* access must fail. If a register is accessible, then \Fsize matching
+* the register's actual size must be supported.
+ */
 #define AC_ACCESS_REGISTER_SIZE_OFFSET      19
 #define AC_ACCESS_REGISTER_SIZE_LENGTH      3
 #define AC_ACCESS_REGISTER_SIZE             (0x7 << AC_ACCESS_REGISTER_SIZE_OFFSET)
+/*
+* When 1, execute the program in the Program Buffer exactly once
+* before performing the read/write.
+ */
 #define AC_ACCESS_REGISTER_PREEXEC_OFFSET   18
 #define AC_ACCESS_REGISTER_PREEXEC_LENGTH   1
 #define AC_ACCESS_REGISTER_PREEXEC          (0x1 << AC_ACCESS_REGISTER_PREEXEC_OFFSET)
+/*
+* When 1, execute the program in the Program Buffer exactly once
+* after performing the read/write.
+ */
 #define AC_ACCESS_REGISTER_POSTEXEC_OFFSET  17
 #define AC_ACCESS_REGISTER_POSTEXEC_LENGTH  1
 #define AC_ACCESS_REGISTER_POSTEXEC         (0x1 << AC_ACCESS_REGISTER_POSTEXEC_OFFSET)
+/*
+* 0: Copy data from \Rdatazero into the specified register.
+*
+* 1: Copy data from the specified register into \Rdatazero.
+*
+* (If XLEN is greater than 32, more {\tt data} registers are involved.)
+ */
 #define AC_ACCESS_REGISTER_WRITE_OFFSET     16
 #define AC_ACCESS_REGISTER_WRITE_LENGTH     1
 #define AC_ACCESS_REGISTER_WRITE            (0x1 << AC_ACCESS_REGISTER_WRITE_OFFSET)
+/*
+* Number of the register to access, as described in Table~\ref{tab:regno}.
+ */
 #define AC_ACCESS_REGISTER_REGNO_OFFSET     0
 #define AC_ACCESS_REGISTER_REGNO_LENGTH     16
 #define AC_ACCESS_REGISTER_REGNO            (0xffff << AC_ACCESS_REGISTER_REGNO_OFFSET)
@@ -19,33 +48,106 @@
 #define AC_QUICK_ACCESS_1_LENGTH            8
 #define AC_QUICK_ACCESS_1                   (0xff << AC_QUICK_ACCESS_1_OFFSET)
 #define CSR_DCSR                            0x7b0
+/*
+* 0: There is no external debug support.
+*
+* 1: External debug support exists as it is described in this document.
+*
+* Other values are reserved for future standards.
+ */
 #define CSR_DCSR_XDEBUGVER_OFFSET           30
 #define CSR_DCSR_XDEBUGVER_LENGTH           2
 #define CSR_DCSR_XDEBUGVER                  (0x3 << CSR_DCSR_XDEBUGVER_OFFSET)
+/*
+* When 1, {\tt ebreak} instructions in Machine Mode enter Halt Mode.
+ */
 #define CSR_DCSR_EBREAKM_OFFSET             15
 #define CSR_DCSR_EBREAKM_LENGTH             1
 #define CSR_DCSR_EBREAKM                    (0x1 << CSR_DCSR_EBREAKM_OFFSET)
+/*
+* When 1, {\tt ebreak} instructions in Hypervisor Mode enter Halt Mode.
+ */
 #define CSR_DCSR_EBREAKH_OFFSET             14
 #define CSR_DCSR_EBREAKH_LENGTH             1
 #define CSR_DCSR_EBREAKH                    (0x1 << CSR_DCSR_EBREAKH_OFFSET)
+/*
+* When 1, {\tt ebreak} instructions in Supervisor Mode enter Halt Mode.
+ */
 #define CSR_DCSR_EBREAKS_OFFSET             13
 #define CSR_DCSR_EBREAKS_LENGTH             1
 #define CSR_DCSR_EBREAKS                    (0x1 << CSR_DCSR_EBREAKS_OFFSET)
+/*
+* When 1, {\tt ebreak} instructions in User/Application Mode enter
+* Halt Mode.
+ */
 #define CSR_DCSR_EBREAKU_OFFSET             12
 #define CSR_DCSR_EBREAKU_LENGTH             1
 #define CSR_DCSR_EBREAKU                    (0x1 << CSR_DCSR_EBREAKU_OFFSET)
+/*
+* Controls the behavior of any counters while the component is in
+* Halt Mode. This includes the {\tt cycle} and {\tt instret} CSRs.
+* When 1, counters are stopped when the component is in Halt Mode.
+* Otherwise, the counters continue to run.
+*
+* An implementation may choose not to support writing to this bit.
+* The debugger must read back the value it writes to check whether
+* the feature is supported.
+ */
 #define CSR_DCSR_STOPCYCLE_OFFSET           10
 #define CSR_DCSR_STOPCYCLE_LENGTH           1
 #define CSR_DCSR_STOPCYCLE                  (0x1 << CSR_DCSR_STOPCYCLE_OFFSET)
+/*
+* Controls the behavior of any timers while the component is in Debug
+* Mode.  This includes the {\tt time} and {tt timeh} CSRs.  When 1,
+* timers are stopped when the component is in Halt Mode.  Otherwise,
+* the timers continue to run.
+*
+* An implementation may choose not to support writing to this bit.
+* The debugger must read back the value it writes to check whether
+* the feature is supported.
+ */
 #define CSR_DCSR_STOPTIME_OFFSET            9
 #define CSR_DCSR_STOPTIME_LENGTH            1
 #define CSR_DCSR_STOPTIME                   (0x1 << CSR_DCSR_STOPTIME_OFFSET)
+/*
+* Explains why Halt Mode was entered.
+*
+* When there are multiple reasons to enter Halt Mode in a single
+* cycle, the cause with the highest priority is the one written.
+*
+* 1: A software breakpoint was hit. (priority 3)
+*
+* 2: The Trigger Module caused a halt. (priority 4)
+*
+* 3: The debug interrupt was asserted by the Debug Module. (priority 2)
+*
+* 4: The hart single stepped because \Fstep was set. (priority 1)
+*
+* 5: \Fhaltreq was set. (priority 0)
+*
+* Other values are reserved for future use.
+ */
 #define CSR_DCSR_CAUSE_OFFSET               6
 #define CSR_DCSR_CAUSE_LENGTH               3
 #define CSR_DCSR_CAUSE                      (0x7 << CSR_DCSR_CAUSE_OFFSET)
+/*
+* When set and not in Halt Mode, the hart will only execute a single
+* instruction, and then enter Halt Mode. Interrupts are disabled
+* when this bit is set.
+ */
 #define CSR_DCSR_STEP_OFFSET                2
 #define CSR_DCSR_STEP_LENGTH                1
 #define CSR_DCSR_STEP                       (0x1 << CSR_DCSR_STEP_OFFSET)
+/*
+* Contains the privilege level the hart was operating in when Debug
+* Mode was entered. The encoding is describe in Table
+* \ref{tab:privlevel}.  A debugger can change this value to change
+* the hart's privilege level when exiting Halt Mode.
+*
+* Not all privilege levels are supported on all harts. If the
+* encoding written is not supported or the debugger is not allowed to
+* change to it, the hart may change to any supported privilege level.
+ */
 #define CSR_DCSR_PRV_OFFSET                 0
 #define CSR_DCSR_PRV_LENGTH                 2
 #define CSR_DCSR_PRV                        (0x3 << CSR_DCSR_PRV_OFFSET)
@@ -56,44 +158,139 @@
 #define CSR_DSCRATCH0                       0x7b2
 #define CSR_DSCRATCH1                       0x7b3
 #define CSR_PRIV                            virtual
+/*
+* Contains the privilege level the hart was operating in when Debug
+* Mode was entered. The encoding is describe in Table
+* \ref{tab:privlevel}. A user can write this value to change the
+* hart's privilege level when exiting Halt Mode.
+ */
 #define CSR_PRIV_PRV_OFFSET                 0
 #define CSR_PRIV_PRV_LENGTH                 2
 #define CSR_PRIV_PRV                        (0x3 << CSR_PRIV_PRV_OFFSET)
 #define DMI_DMCONTROL                       0x00
+/*
+* Halt request signal for the hart selected by \Fhartsel.  Writes
+* apply to the new value of \Fhartsel.
+ */
 #define DMI_DMCONTROL_HALTREQ_OFFSET        31
 #define DMI_DMCONTROL_HALTREQ_LENGTH        1
 #define DMI_DMCONTROL_HALTREQ               (0x1 << DMI_DMCONTROL_HALTREQ_OFFSET)
+/*
+* This bit controls the reset signal from the DM to the rest of the
+* system. To perform a reset the debugger writes 1, and then writes 0
+* to deassert the reset.
+ */
 #define DMI_DMCONTROL_RESET_OFFSET          30
 #define DMI_DMCONTROL_RESET_LENGTH          1
 #define DMI_DMCONTROL_RESET                 (0x1 << DMI_DMCONTROL_RESET_OFFSET)
+/*
+* This bit serves as a reset signal for the Debug Module itself.
+* When 0, the module is held in reset. When 1, it functions normally.
+* No other mechanism should exist that may result in resetting the
+* Debug Module after power up, including the platform's system reset
+* or Debug Transport reset signals.
+*
+* A debugger should pulse this bit low to ensure that the Debug
+* Module is fully reset and ready to use.
+*
+* Implementations may use this bit to aid debugging, for example by
+* preventing the Debug Module from being power gated while debugging
+* is active.
+ */
 #define DMI_DMCONTROL_DMACTIVE_OFFSET       29
 #define DMI_DMCONTROL_DMACTIVE_LENGTH       1
 #define DMI_DMCONTROL_DMACTIVE              (0x1 << DMI_DMCONTROL_DMACTIVE_OFFSET)
+/*
+* The status of the currently selected hart.
+*
+* 0: Halted.
+*
+* 1: Running.
+*
+* 2: Unavailable (eg. powered down, held in reset).
+*
+* 3: \Fhartsel specifies a hart that does not exist in this system.
+ */
 #define DMI_DMCONTROL_HARTSTATUS_OFFSET     26
 #define DMI_DMCONTROL_HARTSTATUS_LENGTH     2
 #define DMI_DMCONTROL_HARTSTATUS            (0x3 << DMI_DMCONTROL_HARTSTATUS_OFFSET)
+/*
+* The DM-specific index of the hart to select.
+ */
 #define DMI_DMCONTROL_HARTSEL_OFFSET        16
 #define DMI_DMCONTROL_HARTSEL_LENGTH        10
 #define DMI_DMCONTROL_HARTSEL               (0x3ff << DMI_DMCONTROL_HARTSEL_OFFSET)
+/*
+* 0 when authentication is required before using the DM.  1 when the
+* authentication check has passed. On components that don't implement
+* authentication, this bit must be preset as 1.
+ */
 #define DMI_DMCONTROL_AUTHENTICATED_OFFSET  7
 #define DMI_DMCONTROL_AUTHENTICATED_LENGTH  1
 #define DMI_DMCONTROL_AUTHENTICATED         (0x1 << DMI_DMCONTROL_AUTHENTICATED_OFFSET)
+/*
+* While 1, writes to \Rauthdatazero and \Rauthdataone may be ignored
+* or may result in authentication failing.  Authentication mechanisms
+* that are slow (or intentionally delayed) must set this bit when
+* they're not ready to process another write.
+ */
 #define DMI_DMCONTROL_AUTHBUSY_OFFSET       6
 #define DMI_DMCONTROL_AUTHBUSY_LENGTH       1
 #define DMI_DMCONTROL_AUTHBUSY              (0x1 << DMI_DMCONTROL_AUTHBUSY_OFFSET)
+/*
+* Defines the kind of authentication required to use this DM.
+*
+* 0: No authentication is required.
+*
+* 1: A password is required.
+*
+* 2: A challenge-response mechanism is in place.
+*
+* 3: Reserved for future use.
+ */
 #define DMI_DMCONTROL_AUTHTYPE_OFFSET       4
 #define DMI_DMCONTROL_AUTHTYPE_LENGTH       2
 #define DMI_DMCONTROL_AUTHTYPE              (0x3 << DMI_DMCONTROL_AUTHTYPE_OFFSET)
+/*
+* 0: There is no Debug Module present.
+*
+* 1: There is a Debug Module and it conforms to version 0.12 of this
+* specification.
+*
+* Other values are reserved for future use.
+ */
 #define DMI_DMCONTROL_VERSION_OFFSET        0
 #define DMI_DMCONTROL_VERSION_LENGTH        4
 #define DMI_DMCONTROL_VERSION               (0xf << DMI_DMCONTROL_VERSION_OFFSET)
 #define DMI_HARTINFO                        0x01
+/*
+* 0: The {\tt data} registers are shadowed in the hart by CSR
+* registers. Each CSR register is XLEN bits in size, and corresponds
+* to a single argument, per Table~\ref{tab:datareg}.
+*
+* 1: The {\tt data} registers are shadowed in the hart's memory map.
+* Each register takes up 4 bytes in the memory map.
+ */
 #define DMI_HARTINFO_DATAACCESS_OFFSET      16
 #define DMI_HARTINFO_DATAACCESS_LENGTH      1
 #define DMI_HARTINFO_DATAACCESS             (0x1 << DMI_HARTINFO_DATAACCESS_OFFSET)
+/*
+* If \Fdataaccess is 0: Number of CSR registers dedicated to
+* shadowing the {\tt data} registers.
+*
+* If \Fdataaccess is 1: Number of 32-bit words in the memory map
+* dedicated to shadowing the {\tt data} registers.
+ */
 #define DMI_HARTINFO_DATASIZE_OFFSET        12
 #define DMI_HARTINFO_DATASIZE_LENGTH        4
 #define DMI_HARTINFO_DATASIZE               (0xf << DMI_HARTINFO_DATASIZE_OFFSET)
+/*
+* If \Fdataaccess is 0: The number of the first CSR dedicated to
+* shadowing the {\tt data} registers.
+*
+* If \Fdataaccess is 1: Signed address of RAM where the {\tt data}
+* registers are shadowed.
+ */
 #define DMI_HARTINFO_DATAADDR_OFFSET        0
 #define DMI_HARTINFO_DATAADDR_LENGTH        12
 #define DMI_HARTINFO_DATAADDR               (0xfff << DMI_HARTINFO_DATAADDR_OFFSET)
@@ -195,67 +392,164 @@
 #define DMI_HALTSUM_HALT31_0_LENGTH         1
 #define DMI_HALTSUM_HALT31_0                (0x1 << DMI_HALTSUM_HALT31_0_OFFSET)
 #define DMI_SBCS                            0x03
+/*
+* When a 1 is written here, triggers a read at the address in {\tt
+* sbaddress} using the access size set by \Fsbaccess.
+ */
 #define DMI_SBCS_SBSINGLEREAD_OFFSET        20
 #define DMI_SBCS_SBSINGLEREAD_LENGTH        1
 #define DMI_SBCS_SBSINGLEREAD               (0x1 << DMI_SBCS_SBSINGLEREAD_OFFSET)
+/*
+* Select the access size to use for system bus accesses triggered by
+* writes to the {\tt sbaddress} registers or \Rsbdatazero.
+*
+* 0: 8-bit
+*
+* 1: 16-bit
+*
+* 2: 32-bit
+*
+* 3: 64-bit
+*
+* 4: 128-bit
+*
+* If an unsupported system bus access size is written here,
+* the DM may not
+* perform the access, or may perform the access with any access size
+ */
 #define DMI_SBCS_SBACCESS_OFFSET            17
 #define DMI_SBCS_SBACCESS_LENGTH            3
 #define DMI_SBCS_SBACCESS                   (0x7 << DMI_SBCS_SBACCESS_OFFSET)
+/*
+* When 1, the internal address value (used by the system bus master)
+* is incremented by the access size (in bytes) selected in \Fsbaccess
+* after every system bus access.
+ */
 #define DMI_SBCS_SBAUTOINCREMENT_OFFSET     16
 #define DMI_SBCS_SBAUTOINCREMENT_LENGTH     1
 #define DMI_SBCS_SBAUTOINCREMENT            (0x1 << DMI_SBCS_SBAUTOINCREMENT_OFFSET)
+/*
+* When 1, every read from \Rsbdatazero automatically triggers a system
+* bus read at the new address.
+ */
 #define DMI_SBCS_SBAUTOREAD_OFFSET          15
 #define DMI_SBCS_SBAUTOREAD_LENGTH          1
 #define DMI_SBCS_SBAUTOREAD                 (0x1 << DMI_SBCS_SBAUTOREAD_OFFSET)
+/*
+* When the debug module's system bus
+* master causes a bus error, this field gets set.
+* It remains set until 0 is written to any bit in this field. Until
+* that happens, the system bus master is busy and no more accesses can be
+* initiated by the debug module.
+*
+* 0: There was no bus error.
+*
+* 1: There was a timeout.
+*
+* 2: A bad address was accessed.
+*
+* 3: There was some other error (eg. alignment).
+*
+* 4: The system bus master was busy when a one of the {\tt sbaddress} or
+* {\tt sbdata} registers was written.
+ */
 #define DMI_SBCS_SBERROR_OFFSET             12
 #define DMI_SBCS_SBERROR_LENGTH             3
 #define DMI_SBCS_SBERROR                    (0x7 << DMI_SBCS_SBERROR_OFFSET)
+/*
+* Width of system bus addresses in bits. (0 indicates there is no bus
+* access support.)
+ */
 #define DMI_SBCS_SBASIZE_OFFSET             5
 #define DMI_SBCS_SBASIZE_LENGTH             7
 #define DMI_SBCS_SBASIZE                    (0x7f << DMI_SBCS_SBASIZE_OFFSET)
+/*
+* 1 when 128-bit system bus accesses are supported.
+ */
 #define DMI_SBCS_SBACCESS128_OFFSET         4
 #define DMI_SBCS_SBACCESS128_LENGTH         1
 #define DMI_SBCS_SBACCESS128                (0x1 << DMI_SBCS_SBACCESS128_OFFSET)
+/*
+* 1 when 64-bit system bus accesses are supported.
+ */
 #define DMI_SBCS_SBACCESS64_OFFSET          3
 #define DMI_SBCS_SBACCESS64_LENGTH          1
 #define DMI_SBCS_SBACCESS64                 (0x1 << DMI_SBCS_SBACCESS64_OFFSET)
+/*
+* 1 when 32-bit system bus accesses are supported.
+ */
 #define DMI_SBCS_SBACCESS32_OFFSET          2
 #define DMI_SBCS_SBACCESS32_LENGTH          1
 #define DMI_SBCS_SBACCESS32                 (0x1 << DMI_SBCS_SBACCESS32_OFFSET)
+/*
+* 1 when 16-bit system bus accesses are supported.
+ */
 #define DMI_SBCS_SBACCESS16_OFFSET          1
 #define DMI_SBCS_SBACCESS16_LENGTH          1
 #define DMI_SBCS_SBACCESS16                 (0x1 << DMI_SBCS_SBACCESS16_OFFSET)
+/*
+* 1 when 8-bit system bus accesses are supported.
+ */
 #define DMI_SBCS_SBACCESS8_OFFSET           0
 #define DMI_SBCS_SBACCESS8_LENGTH           1
 #define DMI_SBCS_SBACCESS8                  (0x1 << DMI_SBCS_SBACCESS8_OFFSET)
 #define DMI_SBADDRESS0                      0x04
+/*
+* Accesses bits 31:0 of the internal address.
+ */
 #define DMI_SBADDRESS0_ADDRESS_OFFSET       0
 #define DMI_SBADDRESS0_ADDRESS_LENGTH       32
 #define DMI_SBADDRESS0_ADDRESS              (0xffffffff << DMI_SBADDRESS0_ADDRESS_OFFSET)
 #define DMI_SBADDRESS1                      0x05
+/*
+* Accesses bits 63:32 of the internal address (if the system address
+* bus is that wide).
+ */
 #define DMI_SBADDRESS1_ADDRESS_OFFSET       0
 #define DMI_SBADDRESS1_ADDRESS_LENGTH       32
 #define DMI_SBADDRESS1_ADDRESS              (0xffffffff << DMI_SBADDRESS1_ADDRESS_OFFSET)
 #define DMI_SBADDRESS2                      0x06
+/*
+* The same as \Fbusy in \Rsbaddresszero.
+ */
 #define DMI_SBADDRESS2_BUSY_OFFSET          31
 #define DMI_SBADDRESS2_BUSY_LENGTH          1
 #define DMI_SBADDRESS2_BUSY                 (0x1 << DMI_SBADDRESS2_BUSY_OFFSET)
+/*
+* Accesses bits 91:61 of the internal address (if the system address
+* bus is that wide).
+ */
 #define DMI_SBADDRESS2_ADDRESS_OFFSET       0
 #define DMI_SBADDRESS2_ADDRESS_LENGTH       31
 #define DMI_SBADDRESS2_ADDRESS              (0x7fffffff << DMI_SBADDRESS2_ADDRESS_OFFSET)
 #define DMI_SBDATA0                         0x07
+/*
+* Accesses bits 31:0 of the internal data.
+ */
 #define DMI_SBDATA0_DATA_OFFSET             0
 #define DMI_SBDATA0_DATA_LENGTH             32
 #define DMI_SBDATA0_DATA                    (0xffffffff << DMI_SBDATA0_DATA_OFFSET)
 #define DMI_SBDATA1                         0x08
+/*
+* Accesses bits 63:32 of the internal data (if the system bus is
+* that wide).
+ */
 #define DMI_SBDATA1_DATA_OFFSET             0
 #define DMI_SBDATA1_DATA_LENGTH             32
 #define DMI_SBDATA1_DATA                    (0xffffffff << DMI_SBDATA1_DATA_OFFSET)
 #define DMI_SBDATA2                         0x09
+/*
+* Accesses bits 95:64 of the internal data (if the system bus is
+* that wide).
+ */
 #define DMI_SBDATA2_DATA_OFFSET             0
 #define DMI_SBDATA2_DATA_LENGTH             32
 #define DMI_SBDATA2_DATA                    (0xffffffff << DMI_SBDATA2_DATA_OFFSET)
 #define DMI_SBDATA3                         0x0a
+/*
+* Accesses bits 127:96 of the internal data (if the system bus is
+* that wide).
+ */
 #define DMI_SBDATA3_DATA_OFFSET             0
 #define DMI_SBDATA3_DATA_LENGTH             32
 #define DMI_SBDATA3_DATA                    (0xffffffff << DMI_SBDATA3_DATA_OFFSET)
@@ -289,15 +583,54 @@
 #define DMI_ABSTRACTCS_AUTOEXEC1_OFFSET     9
 #define DMI_ABSTRACTCS_AUTOEXEC1_LENGTH     1
 #define DMI_ABSTRACTCS_AUTOEXEC1            (0x1 << DMI_ABSTRACTCS_AUTOEXEC1_OFFSET)
+/*
+* When 1, accesses to \Rdatazero cause the command in \Rcommand to be
+* executed again.
+*
+* The same is true for other other autoexec bits: When 1, accesses to
+* {\tt data}N cause the command in \Rcommand to be executed again.
+ */
 #define DMI_ABSTRACTCS_AUTOEXEC0_OFFSET     8
 #define DMI_ABSTRACTCS_AUTOEXEC0_LENGTH     1
 #define DMI_ABSTRACTCS_AUTOEXEC0            (0x1 << DMI_ABSTRACTCS_AUTOEXEC0_OFFSET)
+/*
+* Gets set if an abstract command fails. No abstract command is
+* started until the value is reset to 0.
+*
+* 0 (none): No error.
+*
+* 1 (busy): An abstract command was executing while \Rcommand or one
+* of the {\tt data} registers was accessed.
+*
+* 2 (not supported): The requested command is not supported. A
+* command that is not supported while the hart is running may be
+* supported when it is halted.
+*
+* 3 (exception): An exception occurred while executing the command
+* (eg. while executing the Program Buffer).
+*
+* 4 (halt/resume): An abstract command couldn't execute because the
+* hart wasn't in the expected state (running/halted).
+*
+* 7 (other): The command failed for another reason.
+ */
 #define DMI_ABSTRACTCS_CMDERR_OFFSET        5
 #define DMI_ABSTRACTCS_CMDERR_LENGTH        3
 #define DMI_ABSTRACTCS_CMDERR               (0x7 << DMI_ABSTRACTCS_CMDERR_OFFSET)
+/*
+* 1: An abstract command is currently being executed.
+*
+* This bit is set as soon as \Rcommand is written, and isn't cleared
+* until that command has completed.
+ */
 #define DMI_ABSTRACTCS_BUSY_OFFSET          4
 #define DMI_ABSTRACTCS_BUSY_LENGTH          1
 #define DMI_ABSTRACTCS_BUSY                 (0x1 << DMI_ABSTRACTCS_BUSY_OFFSET)
+/*
+* Number of {\tt data} registers that are implemented as part of the
+* abstract command interface. If it's 0 then no abstract interface is
+* implemented at all.
+ */
 #define DMI_ABSTRACTCS_DATACOUNT_OFFSET     0
 #define DMI_ABSTRACTCS_DATACOUNT_LENGTH     4
 #define DMI_ABSTRACTCS_DATACOUNT            (0xf << DMI_ABSTRACTCS_DATACOUNT_OFFSET)
@@ -325,9 +658,15 @@
 #define DMI_SERDATA_DATA_LENGTH             32
 #define DMI_SERDATA_DATA                    (0xffffffff << DMI_SERDATA_DATA_OFFSET)
 #define DMI_SERSTATUS                       0x1d
+/*
+* Number of supported serial ports.
+ */
 #define DMI_SERSTATUS_SERIALCOUNT_OFFSET    28
 #define DMI_SERSTATUS_SERIALCOUNT_LENGTH    4
 #define DMI_SERSTATUS_SERIALCOUNT           (0xf << DMI_SERSTATUS_SERIALCOUNT_OFFSET)
+/*
+* Select which serial port is accessed by \Rserdata.
+ */
 #define DMI_SERSTATUS_SERIAL_OFFSET         16
 #define DMI_SERSTATUS_SERIAL_LENGTH         3
 #define DMI_SERSTATUS_SERIAL                (0x7 << DMI_SERSTATUS_SERIAL_OFFSET)
@@ -373,13 +712,30 @@
 #define DMI_SERSTATUS_FULL_OVERFLOW1_OFFSET 2
 #define DMI_SERSTATUS_FULL_OVERFLOW1_LENGTH 1
 #define DMI_SERSTATUS_FULL_OVERFLOW1        (0x1 << DMI_SERSTATUS_FULL_OVERFLOW1_OFFSET)
+/*
+* 1 when the core-to-debugger queue for serial port 0 is not empty.
+ */
 #define DMI_SERSTATUS_VALID0_OFFSET         1
 #define DMI_SERSTATUS_VALID0_LENGTH         1
 #define DMI_SERSTATUS_VALID0                (0x1 << DMI_SERSTATUS_VALID0_OFFSET)
+/*
+* 1 when the debugger-to-core queue for serial port 0 is either full,
+* or has overflowed. Overflow state is sticky, and can be reset by
+* writing 0 to this bit.
+ */
 #define DMI_SERSTATUS_FULL_OVERFLOW0_OFFSET 0
 #define DMI_SERSTATUS_FULL_OVERFLOW0_LENGTH 1
 #define DMI_SERSTATUS_FULL_OVERFLOW0        (0x1 << DMI_SERSTATUS_FULL_OVERFLOW0_OFFSET)
 #define DMI_ACCESSCS                        0x1f
+/*
+* Size of the Program Buffer, in 32-bit words. Valid sizes are 0 - 12.
+*
+* A debugger must not access any Instruction Buffer locations that
+* fall outside the range specified here.
+*
+* TODO: Explain what can be done with each size of the buffer, to suggest
+* why you would want more or less words.
+ */
 #define DMI_ACCESSCS_PROGSIZE_OFFSET        0
 #define DMI_ACCESSCS_PROGSIZE_LENGTH        4
 #define DMI_ACCESSCS_PROGSIZE               (0xf << DMI_ACCESSCS_PROGSIZE_OFFSET)
@@ -399,36 +755,68 @@
 #define DMI_IBUF10                          0x2a
 #define DMI_IBUF11                          0x2b
 #define SERINFO                             0x110
+/*
+* Like \Fserialzero.
+ */
 #define SERINFO_SERIAL7_OFFSET              7
 #define SERINFO_SERIAL7_LENGTH              1
 #define SERINFO_SERIAL7                     (0x1 << SERINFO_SERIAL7_OFFSET)
+/*
+* Like \Fserialzero.
+ */
 #define SERINFO_SERIAL6_OFFSET              6
 #define SERINFO_SERIAL6_LENGTH              1
 #define SERINFO_SERIAL6                     (0x1 << SERINFO_SERIAL6_OFFSET)
+/*
+* Like \Fserialzero.
+ */
 #define SERINFO_SERIAL5_OFFSET              5
 #define SERINFO_SERIAL5_LENGTH              1
 #define SERINFO_SERIAL5                     (0x1 << SERINFO_SERIAL5_OFFSET)
+/*
+* Like \Fserialzero.
+ */
 #define SERINFO_SERIAL4_OFFSET              4
 #define SERINFO_SERIAL4_LENGTH              1
 #define SERINFO_SERIAL4                     (0x1 << SERINFO_SERIAL4_OFFSET)
+/*
+* Like \Fserialzero.
+ */
 #define SERINFO_SERIAL3_OFFSET              3
 #define SERINFO_SERIAL3_LENGTH              1
 #define SERINFO_SERIAL3                     (0x1 << SERINFO_SERIAL3_OFFSET)
+/*
+* Like \Fserialzero.
+ */
 #define SERINFO_SERIAL2_OFFSET              2
 #define SERINFO_SERIAL2_LENGTH              1
 #define SERINFO_SERIAL2                     (0x1 << SERINFO_SERIAL2_OFFSET)
+/*
+* Like \Fserialzero.
+ */
 #define SERINFO_SERIAL1_OFFSET              1
 #define SERINFO_SERIAL1_LENGTH              1
 #define SERINFO_SERIAL1                     (0x1 << SERINFO_SERIAL1_OFFSET)
+/*
+* 1 means serial interface 0 is supported.
+ */
 #define SERINFO_SERIAL0_OFFSET              0
 #define SERINFO_SERIAL0_LENGTH              1
 #define SERINFO_SERIAL0                     (0x1 << SERINFO_SERIAL0_OFFSET)
 #define SERSEND0                            0x200
 #define SERRECV0                            0x204
 #define SERSTAT0                            0x208
+/*
+* Send ready. 1 when the core-to-debugger queue is not full. 0
+* otherwise.
+ */
 #define SERSTAT0_SENDR_OFFSET               1
 #define SERSTAT0_SENDR_LENGTH               1
 #define SERSTAT0_SENDR                      (0x1 << SERSTAT0_SENDR_OFFSET)
+/*
+* Receive ready. 1 when the debugger-to-core queue is not empty. 0
+* otherwise.
+ */
 #define SERSTAT0_RECVR_OFFSET               0
 #define SERSTAT0_RECVR_LENGTH               1
 #define SERSTAT0_RECVR                      (0x1 << SERSTAT0_RECVR_OFFSET)
@@ -458,12 +846,39 @@
 #define CSR_TSELECT_INDEX_LENGTH            XLEN
 #define CSR_TSELECT_INDEX                   (((1L<<XLEN)-1) << CSR_TSELECT_INDEX_OFFSET)
 #define CSR_TDATA1                          0x7a1
+/*
+* 0: There is no trigger at this \Rtselect.
+*
+* 1: The trigger is a legacy SiFive address match trigger. These
+* should not be implemented and aren't further documented here.
+*
+* 2: The trigger is an address/data match trigger.
+*
+* 3: The trigger is an instruction count trigger.
+*
+* 15: This trigger exists (so enumeration shouldn't terminate), but
+* is not currently available.
+*
+* Other values are reserved for future use.
+ */
 #define CSR_TDATA1_TYPE_OFFSET              XLEN-4
 #define CSR_TDATA1_TYPE_LENGTH              4
-#define CSR_TDATA1_TYPE                     (0xf << CSR_TDATA1_TYPE_OFFSET)
+#define CSR_TDATA1_TYPE                     (0xfL << CSR_TDATA1_TYPE_OFFSET)
+/*
+* 0: Both Debug and M Mode can write the {\tt tdata} registers at the
+* selected \Rtselect.
+*
+* 1: Only Debug Mode can write the {\tt tdata} registers at the
+* selected \Rtselect.  Writes from other modes are ignored.
+*
+* This bit is only writable from Debug Mode.
+ */
 #define CSR_TDATA1_DMODE_OFFSET             XLEN-5
 #define CSR_TDATA1_DMODE_LENGTH             1
-#define CSR_TDATA1_DMODE                    (0x1 << CSR_TDATA1_DMODE_OFFSET)
+#define CSR_TDATA1_DMODE                    (0x1L << CSR_TDATA1_DMODE_OFFSET)
+/*
+* Trigger-specific data.
+ */
 #define CSR_TDATA1_DATA_OFFSET              0
 #define CSR_TDATA1_DATA_LENGTH              XLEN - 5
 #define CSR_TDATA1_DATA                     (((1L<<XLEN - 5)-1) << CSR_TDATA1_DATA_OFFSET)
@@ -478,81 +893,240 @@
 #define CSR_MCONTROL                        0x7a1
 #define CSR_MCONTROL_TYPE_OFFSET            XLEN-4
 #define CSR_MCONTROL_TYPE_LENGTH            4
-#define CSR_MCONTROL_TYPE                   (0xf << CSR_MCONTROL_TYPE_OFFSET)
+#define CSR_MCONTROL_TYPE                   (0xfL << CSR_MCONTROL_TYPE_OFFSET)
 #define CSR_MCONTROL_DMODE_OFFSET           XLEN-5
 #define CSR_MCONTROL_DMODE_LENGTH           1
-#define CSR_MCONTROL_DMODE                  (0x1 << CSR_MCONTROL_DMODE_OFFSET)
+#define CSR_MCONTROL_DMODE                  (0x1L << CSR_MCONTROL_DMODE_OFFSET)
+/*
+* Specifies the largest naturally aligned powers-of-two (NAPOT) range
+* supported by the hardware. The value is the logarithm base 2 of the
+* number of bytes in that range.  A value of 0 indicates that only
+* exact value matches are supported (one byte range). A value of 63
+* corresponds to the maximum NAPOT range, which is $2^{63}$ bytes in
+* size.
+ */
 #define CSR_MCONTROL_MASKMAX_OFFSET         XLEN-11
 #define CSR_MCONTROL_MASKMAX_LENGTH         6
-#define CSR_MCONTROL_MASKMAX                (0x3f << CSR_MCONTROL_MASKMAX_OFFSET)
+#define CSR_MCONTROL_MASKMAX                (0x3fL << CSR_MCONTROL_MASKMAX_OFFSET)
+/*
+* 0: Perform a match on the address.
+*
+* 1: Perform a match on the data value loaded/stored, or the
+* instruction executed.
+ */
 #define CSR_MCONTROL_SELECT_OFFSET          19
 #define CSR_MCONTROL_SELECT_LENGTH          1
-#define CSR_MCONTROL_SELECT                 (0x1 << CSR_MCONTROL_SELECT_OFFSET)
+#define CSR_MCONTROL_SELECT                 (0x1L << CSR_MCONTROL_SELECT_OFFSET)
+/*
+* 0: The action for this trigger will be taken just before the
+* instruction that triggered it is executed, but after all preceding
+* instructions are are committed.
+*
+* 1: The action for this trigger will be taken after the instruction
+* that triggered it is executed. It should be taken before the next
+* instruction is executed, but it is better to implement triggers and
+* not implement that suggestion than to not implement them at all.
+*
+* Most hardware will only implement one timing or the other, possibly
+* dependent on \Fselect, \Fexecute, \Fload, and \Fstore. This bit
+* primarily exists for the hardware to communicate to the debugger
+* what will happen. Hardware may implement the bit fully writable, in
+* which case the debugger has a little more control.
+*
+* Data load triggers with \Ftiming of 0 will result in the same load
+* happening again when the debugger lets the core run. For data load
+* triggers debuggers must first attempt to set the breakpoint with
+* \Ftiming of 1.
+*
+* A chain of triggers that don't all have the same \Ftiming value
+* will never fire (unless consecutive instructions match the
+* appropriate triggers).
+ */
 #define CSR_MCONTROL_TIMING_OFFSET          18
 #define CSR_MCONTROL_TIMING_LENGTH          1
-#define CSR_MCONTROL_TIMING                 (0x1 << CSR_MCONTROL_TIMING_OFFSET)
+#define CSR_MCONTROL_TIMING                 (0x1L << CSR_MCONTROL_TIMING_OFFSET)
+/*
+* Determines what happens when this trigger matches.
+*
+* 0: Raise a breakpoint exception. (Used when software wants to use
+* the trigger module without an external debugger attached.)
+*
+* 1: Enter Debug Mode. (Only supported when \Fdmode is 1.)
+*
+* 2: Start tracing.
+*
+* 3: Stop tracing.
+*
+* 4: Emit trace data for this match. If it is a data access match,
+* emit appropriate Load/Store Address/Data. If it is an instruction
+* execution, emit its PC.
+*
+* Other values are reserved for future use.
+ */
 #define CSR_MCONTROL_ACTION_OFFSET          12
 #define CSR_MCONTROL_ACTION_LENGTH          6
-#define CSR_MCONTROL_ACTION                 (0x3f << CSR_MCONTROL_ACTION_OFFSET)
+#define CSR_MCONTROL_ACTION                 (0x3fL << CSR_MCONTROL_ACTION_OFFSET)
+/*
+* 0: When this trigger matches, the configured action is taken.
+*
+* 1: While this trigger does not match, it prevents the trigger with
+* the next index from matching.
+ */
 #define CSR_MCONTROL_CHAIN_OFFSET           11
 #define CSR_MCONTROL_CHAIN_LENGTH           1
-#define CSR_MCONTROL_CHAIN                  (0x1 << CSR_MCONTROL_CHAIN_OFFSET)
+#define CSR_MCONTROL_CHAIN                  (0x1L << CSR_MCONTROL_CHAIN_OFFSET)
+/*
+* 0: Matches when the value equals \Rtdatatwo.
+*
+* 1: Matches when the top M bits of the value match the top M bits of
+* \Rtdatatwo. M is XLEN-1 minus the index of the least-significant
+* bit containing 0 in \Rtdatatwo.
+*
+* 2: Matches when the value is greater than or equal to \Rtdatatwo.
+*
+* 3: Matches when the value is less than \Rtdatatwo.
+*
+* 4: Matches when the lower half of the value equals the lower half
+* of \Rtdatatwo after the lower half of the value is ANDed with the
+* upper half of \Rtdatatwo.
+*
+* 5: Matches when the upper half of the value equals the lower half
+* of \Rtdatatwo after the upper half of the value is ANDed with the
+* upper half of \Rtdatatwo.
+*
+* Other values are reserved for future use.
+ */
 #define CSR_MCONTROL_MATCH_OFFSET           7
 #define CSR_MCONTROL_MATCH_LENGTH           4
-#define CSR_MCONTROL_MATCH                  (0xf << CSR_MCONTROL_MATCH_OFFSET)
+#define CSR_MCONTROL_MATCH                  (0xfL << CSR_MCONTROL_MATCH_OFFSET)
+/*
+* When set, enable this trigger in M mode.
+ */
 #define CSR_MCONTROL_M_OFFSET               6
 #define CSR_MCONTROL_M_LENGTH               1
-#define CSR_MCONTROL_M                      (0x1 << CSR_MCONTROL_M_OFFSET)
+#define CSR_MCONTROL_M                      (0x1L << CSR_MCONTROL_M_OFFSET)
+/*
+* When set, enable this trigger in H mode.
+ */
 #define CSR_MCONTROL_H_OFFSET               5
 #define CSR_MCONTROL_H_LENGTH               1
-#define CSR_MCONTROL_H                      (0x1 << CSR_MCONTROL_H_OFFSET)
+#define CSR_MCONTROL_H                      (0x1L << CSR_MCONTROL_H_OFFSET)
+/*
+* When set, enable this trigger in S mode.
+ */
 #define CSR_MCONTROL_S_OFFSET               4
 #define CSR_MCONTROL_S_LENGTH               1
-#define CSR_MCONTROL_S                      (0x1 << CSR_MCONTROL_S_OFFSET)
+#define CSR_MCONTROL_S                      (0x1L << CSR_MCONTROL_S_OFFSET)
+/*
+* When set, enable this trigger in U mode.
+ */
 #define CSR_MCONTROL_U_OFFSET               3
 #define CSR_MCONTROL_U_LENGTH               1
-#define CSR_MCONTROL_U                      (0x1 << CSR_MCONTROL_U_OFFSET)
+#define CSR_MCONTROL_U                      (0x1L << CSR_MCONTROL_U_OFFSET)
+/*
+* When set, the trigger fires on the address or opcode of an
+* instruction that is executed.
+ */
 #define CSR_MCONTROL_EXECUTE_OFFSET         2
 #define CSR_MCONTROL_EXECUTE_LENGTH         1
-#define CSR_MCONTROL_EXECUTE                (0x1 << CSR_MCONTROL_EXECUTE_OFFSET)
+#define CSR_MCONTROL_EXECUTE                (0x1L << CSR_MCONTROL_EXECUTE_OFFSET)
+/*
+* When set, the trigger fires on the address or data of a store.
+ */
 #define CSR_MCONTROL_STORE_OFFSET           1
 #define CSR_MCONTROL_STORE_LENGTH           1
-#define CSR_MCONTROL_STORE                  (0x1 << CSR_MCONTROL_STORE_OFFSET)
+#define CSR_MCONTROL_STORE                  (0x1L << CSR_MCONTROL_STORE_OFFSET)
+/*
+* When set, the trigger fires on the address or data of a load.
+ */
 #define CSR_MCONTROL_LOAD_OFFSET            0
 #define CSR_MCONTROL_LOAD_LENGTH            1
-#define CSR_MCONTROL_LOAD                   (0x1 << CSR_MCONTROL_LOAD_OFFSET)
+#define CSR_MCONTROL_LOAD                   (0x1L << CSR_MCONTROL_LOAD_OFFSET)
 #define CSR_ICOUNT                          0x7a1
 #define CSR_ICOUNT_TYPE_OFFSET              XLEN-4
 #define CSR_ICOUNT_TYPE_LENGTH              4
-#define CSR_ICOUNT_TYPE                     (0xf << CSR_ICOUNT_TYPE_OFFSET)
+#define CSR_ICOUNT_TYPE                     (0xfL << CSR_ICOUNT_TYPE_OFFSET)
 #define CSR_ICOUNT_DMODE_OFFSET             XLEN-5
 #define CSR_ICOUNT_DMODE_LENGTH             1
-#define CSR_ICOUNT_DMODE                    (0x1 << CSR_ICOUNT_DMODE_OFFSET)
+#define CSR_ICOUNT_DMODE                    (0x1L << CSR_ICOUNT_DMODE_OFFSET)
+/*
+* When count is decremented to 0, the trigger fires. Instead of
+* changing \Fcount from 1 to 0, it is also acceptable for hardware to
+* clear \Fm, \Fh, \Fs, and \Fu. This allows \Fcount to be hard-wired
+* to 1 if this register just exists for single step.
+ */
 #define CSR_ICOUNT_COUNT_OFFSET             10
 #define CSR_ICOUNT_COUNT_LENGTH             14
-#define CSR_ICOUNT_COUNT                    (0x3fff << CSR_ICOUNT_COUNT_OFFSET)
+#define CSR_ICOUNT_COUNT                    (0x3fffL << CSR_ICOUNT_COUNT_OFFSET)
+/*
+* When set, every instruction completed in M mode decrements \Fcount
+* by 1.
+ */
 #define CSR_ICOUNT_M_OFFSET                 9
 #define CSR_ICOUNT_M_LENGTH                 1
-#define CSR_ICOUNT_M                        (0x1 << CSR_ICOUNT_M_OFFSET)
+#define CSR_ICOUNT_M                        (0x1L << CSR_ICOUNT_M_OFFSET)
+/*
+* When set, every instruction completed in H mode decrements \Fcount
+* by 1.
+ */
 #define CSR_ICOUNT_H_OFFSET                 8
 #define CSR_ICOUNT_H_LENGTH                 1
-#define CSR_ICOUNT_H                        (0x1 << CSR_ICOUNT_H_OFFSET)
+#define CSR_ICOUNT_H                        (0x1L << CSR_ICOUNT_H_OFFSET)
+/*
+* When set, every instruction completed in S mode decrements \Fcount
+* by 1.
+ */
 #define CSR_ICOUNT_S_OFFSET                 7
 #define CSR_ICOUNT_S_LENGTH                 1
-#define CSR_ICOUNT_S                        (0x1 << CSR_ICOUNT_S_OFFSET)
+#define CSR_ICOUNT_S                        (0x1L << CSR_ICOUNT_S_OFFSET)
+/*
+* When set, every instruction completed in U mode decrements \Fcount
+* by 1.
+ */
 #define CSR_ICOUNT_U_OFFSET                 6
 #define CSR_ICOUNT_U_LENGTH                 1
-#define CSR_ICOUNT_U                        (0x1 << CSR_ICOUNT_U_OFFSET)
+#define CSR_ICOUNT_U                        (0x1L << CSR_ICOUNT_U_OFFSET)
+/*
+* Determines what happens when this trigger matches.
+*
+* 0: Raise a debug exception. (Used when software wants to use the
+* trigger module without an external debugger attached.)
+*
+* 1: Enter Debug Mode. (Only supported when \Fdmode is 1.)
+*
+* 2: Start tracing.
+*
+* 3: Stop tracing.
+*
+* 4: Emit trace data for this match. If it is a data access match,
+* emit appropriate Load/Store Address/Data. If it is an instruction
+* execution, emit its PC.
+*
+* Other values are reserved for future use.
+ */
 #define CSR_ICOUNT_ACTION_OFFSET            0
 #define CSR_ICOUNT_ACTION_LENGTH            6
-#define CSR_ICOUNT_ACTION                   (0x3f << CSR_ICOUNT_ACTION_OFFSET)
+#define CSR_ICOUNT_ACTION                   (0x3fL << CSR_ICOUNT_ACTION_OFFSET)
 #define DTM_IDCODE                          0x01
+/*
+* Identifies the release version of this part.
+ */
 #define DTM_IDCODE_VERSION_OFFSET           28
 #define DTM_IDCODE_VERSION_LENGTH           4
 #define DTM_IDCODE_VERSION                  (0xf << DTM_IDCODE_VERSION_OFFSET)
+/*
+* Identifies the designer's part number of this part.
+ */
 #define DTM_IDCODE_PARTNUMBER_OFFSET        12
 #define DTM_IDCODE_PARTNUMBER_LENGTH        16
 #define DTM_IDCODE_PARTNUMBER               (0xffff << DTM_IDCODE_PARTNUMBER_OFFSET)
+/*
+* Identifies the designer/manufacturer of this part. Bits 6:0 must be
+* bits 6:0 of the designer/manufacturer's Identification Code as
+* assigned by JEDEC Standard JEP106. Bits 10:7 contain the modulo-16
+* count of the number of continuation characters (0x7f) in that same
+* Identification Code.
+ */
 #define DTM_IDCODE_MANUFID_OFFSET           1
 #define DTM_IDCODE_MANUFID_LENGTH           11
 #define DTM_IDCODE_MANUFID                  (0x7ff << DTM_IDCODE_MANUFID_OFFSET)
@@ -572,72 +1146,223 @@
 #define DTM_INIT__SETUP__CLAMP              0x0c
 #define DTM_INIT__RUN                       0x0d
 #define DTM_DTMCONTROL                      0x10
+/*
+* Writing 1 to this bit resets the DMI controller, clearing any
+* sticky error state.
+ */
 #define DTM_DTMCONTROL_DMIRESET_OFFSET      16
 #define DTM_DTMCONTROL_DMIRESET_LENGTH      1
 #define DTM_DTMCONTROL_DMIRESET             (0x1 << DTM_DTMCONTROL_DMIRESET_OFFSET)
+/*
+* This is the minimum number of cycles a debugger should spend in
+* Run-Test/Idle after every DMI scan to avoid a 'busy'
+* return code (\Fdmistat of 3). A debugger must still
+* check \Fdmistat when necessary.
+*
+* 0: It is not necessary to enter Run-Test/Idle at all.
+*
+* 1: Enter Run-Test/Idle and leave it immediately.
+*
+* 2: Enter Run-Test/Idle and stay there for 1 cycle before leaving.
+*
+* And so on.
+ */
 #define DTM_DTMCONTROL_IDLE_OFFSET          12
 #define DTM_DTMCONTROL_IDLE_LENGTH          3
 #define DTM_DTMCONTROL_IDLE                 (0x7 << DTM_DTMCONTROL_IDLE_OFFSET)
+/*
+* 0: No error.
+*
+* 1: Reserved. Interpret the same as 2.
+*
+* 2: An operation failed (resulted in \Fop of 2).
+*
+* 3: An operation was attempted while a DMI access was still in
+* progress (resulted in \Fop of 3).
+ */
 #define DTM_DTMCONTROL_DMISTAT_OFFSET       10
 #define DTM_DTMCONTROL_DMISTAT_LENGTH       2
 #define DTM_DTMCONTROL_DMISTAT              (0x3 << DTM_DTMCONTROL_DMISTAT_OFFSET)
+/*
+* The size of \Faddress in \Rdmi.
+ */
 #define DTM_DTMCONTROL_ABITS_OFFSET         4
 #define DTM_DTMCONTROL_ABITS_LENGTH         6
 #define DTM_DTMCONTROL_ABITS                (0x3f << DTM_DTMCONTROL_ABITS_OFFSET)
+/*
+* 0: Version described in spec version 0.11.
+*
+* 1: Version described in spec version 0.12 (and later?), which
+* reduces the DMI data width to 32 bits.
+*
+* Other values are reserved for future use.
+ */
 #define DTM_DTMCONTROL_VERSION_OFFSET       0
 #define DTM_DTMCONTROL_VERSION_LENGTH       4
 #define DTM_DTMCONTROL_VERSION              (0xf << DTM_DTMCONTROL_VERSION_OFFSET)
 #define DTM_DMI                             0x11
+/*
+* Address used for DMI access. In Update-DR this value is used
+* to access the DM over the DMI.
+ */
 #define DTM_DMI_ADDRESS_OFFSET              34
 #define DTM_DMI_ADDRESS_LENGTH              abits
 #define DTM_DMI_ADDRESS                     (((1L<<abits)-1) << DTM_DMI_ADDRESS_OFFSET)
+/*
+* The data to send to the DM over the DMI during Update-DR, and
+* the data returned from the DM as a result of the previous operation.
+ */
 #define DTM_DMI_DATA_OFFSET                 2
 #define DTM_DMI_DATA_LENGTH                 32
-#define DTM_DMI_DATA                        (0xffffffff << DTM_DMI_DATA_OFFSET)
+#define DTM_DMI_DATA                        (0xffffffffL << DTM_DMI_DATA_OFFSET)
+/*
+* When the debugger writes this field, it has the following meaning:
+*
+* 0: Ignore \Fdata. (nop)
+*
+* 1: Read from \Faddress. (read)
+*
+* 2: Write \Fdata to \Faddress. (write)
+*
+* 3: Reserved.
+*
+* When the debugger reads this field, it means the following:
+*
+* 0: The previous operation completed successfully.
+*
+* 1: Reserved.
+*
+* 2: The previous operation returned a non-zero value in \Fop.
+* The data scanned into \Rdmi in this access will be ignored.
+* This status is sticky and can be cleared by writing \Fdmireset
+* in \Rdtmcontrol.
+*
+* (This indicates that the DM itself responded with an error, e.g.
+* in the System Bus and Serial Port overflow/underflow cases.
+* Generally this means that for this type of DM access, the DTM should
+* allow more time between Update-DR and Capture-DR. The most portable way
+* to achieve this is to spend more TCK ticks in Run-Test/Idle state
+* for similar operations.)
+*
+* 3: The previous DMI request is still in progress. The data scanned
+* into \Rdmi in this access will be ignored. This status is sticky
+* and can be cleared by writing \Fdmireset in \Rdtmcontrol. If a
+* debugger sees this status, it needs to give the target more TCK
+* edges between Update-DR and Capture-DR. The simplest way
+* to do that is to add extra transitions in Run-Test/Idle.
+*
+* (The DTM, DM, and/or component may be in different clock domains,
+* so synchronization may be required. Some relatively fixed number of
+* TCK ticks may be needed for the request to reach the DM, complete,
+* and for the response to be synchronized back into the TCK domain.
+* This status is intended to cover these cases, and is orthogonal to
+* the causes for case 2.)
+ */
 #define DTM_DMI_OP_OFFSET                   0
 #define DTM_DMI_OP_LENGTH                   2
-#define DTM_DMI_OP                          (0x3 << DTM_DMI_OP_OFFSET)
+#define DTM_DMI_OP                          (0x3L << DTM_DMI_OP_OFFSET)
 #define SHORTNAME                           0x123
+/*
+* Description of what this field is used for.
+ */
 #define SHORTNAME_FIELD_OFFSET              0
 #define SHORTNAME_FIELD_LENGTH              8
 #define SHORTNAME_FIELD                     (0xff << SHORTNAME_FIELD_OFFSET)
 #define TRACE                               0x728
+/*
+* 1 if the trace buffer has wrapped since the last time \Fdiscard was
+* written. 0 otherwise.
+ */
 #define TRACE_WRAPPED_OFFSET                24
 #define TRACE_WRAPPED_LENGTH                1
 #define TRACE_WRAPPED                       (0x1 << TRACE_WRAPPED_OFFSET)
+/*
+* Emit Timestamp trace sequences.
+ */
 #define TRACE_EMITTIMESTAMP_OFFSET          23
 #define TRACE_EMITTIMESTAMP_LENGTH          1
 #define TRACE_EMITTIMESTAMP                 (0x1 << TRACE_EMITTIMESTAMP_OFFSET)
+/*
+* Emit Store Data trace sequences.
+ */
 #define TRACE_EMITSTOREDATA_OFFSET          22
 #define TRACE_EMITSTOREDATA_LENGTH          1
 #define TRACE_EMITSTOREDATA                 (0x1 << TRACE_EMITSTOREDATA_OFFSET)
+/*
+* Emit Load Data trace sequences.
+ */
 #define TRACE_EMITLOADDATA_OFFSET           21
 #define TRACE_EMITLOADDATA_LENGTH           1
 #define TRACE_EMITLOADDATA                  (0x1 << TRACE_EMITLOADDATA_OFFSET)
+/*
+* Emit Store Address trace sequences.
+ */
 #define TRACE_EMITSTOREADDR_OFFSET          20
 #define TRACE_EMITSTOREADDR_LENGTH          1
 #define TRACE_EMITSTOREADDR                 (0x1 << TRACE_EMITSTOREADDR_OFFSET)
+/*
+* Emit Load Address trace sequences.
+ */
 #define TRACE_EMITLOADADDR_OFFSET           19
 #define TRACE_EMITLOADADDR_LENGTH           1
 #define TRACE_EMITLOADADDR                  (0x1 << TRACE_EMITLOADADDR_OFFSET)
+/*
+* Emit Privilege Level trace sequences.
+ */
 #define TRACE_EMITPRIV_OFFSET               18
 #define TRACE_EMITPRIV_LENGTH               1
 #define TRACE_EMITPRIV                      (0x1 << TRACE_EMITPRIV_OFFSET)
+/*
+* Emit Branch Taken and Branch Not Taken trace sequences.
+ */
 #define TRACE_EMITBRANCH_OFFSET             17
 #define TRACE_EMITBRANCH_LENGTH             1
 #define TRACE_EMITBRANCH                    (0x1 << TRACE_EMITBRANCH_OFFSET)
+/*
+* Emit PC trace sequences.
+ */
 #define TRACE_EMITPC_OFFSET                 16
 #define TRACE_EMITPC_LENGTH                 1
 #define TRACE_EMITPC                        (0x1 << TRACE_EMITPC_OFFSET)
+/*
+* Determine what happens when the trace buffer is full.  0 means wrap
+* and overwrite. 1 means turn off trace until \Fdiscard is written as 1.
+* 2 means cause a trace full exception. 3 is reserved for future use.
+ */
 #define TRACE_FULLACTION_OFFSET             8
 #define TRACE_FULLACTION_LENGTH             2
 #define TRACE_FULLACTION                    (0x3 << TRACE_FULLACTION_OFFSET)
+/*
+* 0: Trace to a dedicated on-core RAM (which is not further defined in
+* this spec).
+*
+* 1: Trace to RAM on the system bus.
+*
+* 2: Send trace data to a dedicated off-chip interface (which is not
+* defined in this spec). This does not affect execution speed.
+*
+* 3: Reserved for future use.
+*
+* Options 0 and 1 slow down execution (eg. because of system bus
+* contention).
+ */
 #define TRACE_DESTINATION_OFFSET            4
 #define TRACE_DESTINATION_LENGTH            2
 #define TRACE_DESTINATION                   (0x3 << TRACE_DESTINATION_OFFSET)
+/*
+* When 1, the trace logic may stall processor execution to ensure it
+* can emit all the trace sequences required. When 0 individual trace
+* sequences may be dropped.
+ */
 #define TRACE_STALL_OFFSET                  2
 #define TRACE_STALL_LENGTH                  1
 #define TRACE_STALL                         (0x1 << TRACE_STALL_OFFSET)
+/*
+* Writing 1 to this bit tells the trace logic that any trace
+* collected is no longer required. When tracing to RAM, it resets the
+* trace write pointer to the start of the memory, as well as
+* \Fwrapped.
+ */
 #define TRACE_DISCARD_OFFSET                1
 #define TRACE_DISCARD_LENGTH                1
 #define TRACE_DISCARD                       (0x1 << TRACE_DISCARD_OFFSET)
diff --git a/riscv/debug_module.cc b/riscv/debug_module.cc
index 551db17..f672a89 100644
--- a/riscv/debug_module.cc
+++ b/riscv/debug_module.cc
@@ -72,7 +72,9 @@ void debug_module_data_t::write32(reg_t addr, uint32_t value)
 
 ///////////////////////// debug_module_t
 
-debug_module_t::debug_module_t(sim_t *sim) : sim(sim)
+debug_module_t::debug_module_t(sim_t *sim) : sim(sim),
+  next_action(jal(ZERO, 0)),
+  action_executed(false)
 {
   dmcontrol = {0};
   dmcontrol.version = 1;
@@ -82,10 +84,7 @@ debug_module_t::debug_module_t(sim_t *sim) : sim(sim)
     halted[i] = false;
   }
 
-  for (unsigned i = 0; i < progsize; i++) {
-    ibuf[i] = 0;
-  }
-
+  memset(program_buffer, 0, sizeof(program_buffer));
 }
 
 void debug_module_t::reset()
@@ -118,19 +117,26 @@ bool debug_module_t::load(reg_t addr, size_t len, uint8_t* bytes)
 
   if (addr >= DEBUG_ROM_ENTRY &&
       addr < DEBUG_ROM_ENTRY + DEBUG_ROM_ENTRY_SIZE) {
-    halted[(addr - DEBUG_ROM_ENTRY) / 4] = true;
-    memcpy(bytes, debug_rom_entry + addr - DEBUG_ROM_ENTRY, len);
 
     if (read32(debug_rom_entry, dmcontrol.hartsel) == jal(ZERO, 0)) {
       // We're here in an infinite loop. That means that whatever abstract
       // command has complete.
       abstractcs.busy = false;
     }
+
+    action_executed = true;
+
+    halted[(addr - DEBUG_ROM_ENTRY) / 4] = true;
+    memcpy(bytes, debug_rom_entry + addr - DEBUG_ROM_ENTRY, len);
     return true;
   }
 
-  // Restore the jump-to-self loop.
-  write32(debug_rom_entry, dmcontrol.hartsel, jal(ZERO, 0));
+  if (action_executed) {
+    // Restore the jump-to-self loop.
+    write32(debug_rom_entry, dmcontrol.hartsel, next_action);
+    next_action = jal(ZERO, 0);
+    action_executed = false;
+  }
 
   if (addr >= DEBUG_ROM_CODE &&
       addr < DEBUG_ROM_CODE + DEBUG_ROM_CODE_SIZE) {
@@ -138,6 +144,11 @@ bool debug_module_t::load(reg_t addr, size_t len, uint8_t* bytes)
     return true;
   }
 
+  if (addr >= DEBUG_RAM_START && addr < DEBUG_RAM_END) {
+    memcpy(bytes, program_buffer + addr - DEBUG_RAM_START, len);
+    return true;
+  }
+
   if (addr >= DEBUG_ROM_EXCEPTION &&
       addr < DEBUG_ROM_EXCEPTION + DEBUG_ROM_EXCEPTION_SIZE) {
     memcpy(bytes, debug_rom_exception + addr - DEBUG_ROM_EXCEPTION, len);
@@ -157,6 +168,11 @@ bool debug_module_t::store(reg_t addr, size_t len, const uint8_t* bytes)
 {
   addr = DEBUG_START + addr;
 
+  if (addr >= DEBUG_RAM_START && addr < DEBUG_RAM_END) {
+    memcpy(program_buffer + addr - DEBUG_RAM_START, bytes, len);
+    return true;
+  }
+
   fprintf(stderr, "ERROR: invalid store to debug module: %zd bytes at 0x%016"
           PRIx64 "\n", len, addr);
   return false;
@@ -198,7 +214,7 @@ bool debug_module_t::dmi_read(unsigned address, uint32_t *value)
   if (address >= DMI_DATA0 && address < DMI_DATA0 + abstractcs.datacount) {
     result = dmdata.read32(4 * (address - DMI_DATA0));
   } else if (address >= DMI_IBUF0 && address < DMI_IBUF0 + progsize) {
-    result = ibuf[address - DMI_IBUF0];
+    result = read32(program_buffer, address - DMI_IBUF0);
   } else {
     switch (address) {
       case DMI_DMCONTROL:
@@ -306,10 +322,22 @@ bool debug_module_t::perform_abstract_command(uint32_t command)
         abstractcs.cmderr = abstractcs.CMDERR_NOTSUP;
         return true;
     }
-    write32(debug_rom_code, 1, ebreak());
+    if (get_field(command, AC_ACCESS_REGISTER_POSTEXEC)) {
+      write32(debug_rom_code, 1, jal(ZERO, DEBUG_RAM_START - DEBUG_ROM_CODE - 4));
+    } else {
+      write32(debug_rom_code, 1, ebreak());
+    }
+
+    if (get_field(command, AC_ACCESS_REGISTER_PREEXEC)) {
+      write32(debug_rom_entry, dmcontrol.hartsel,
+          jal(ZERO, DEBUG_RAM_START - (DEBUG_ROM_ENTRY + 4 * dmcontrol.hartsel)));
+      next_action =
+        jal(ZERO, DEBUG_ROM_CODE - (DEBUG_ROM_ENTRY + 4 * dmcontrol.hartsel));
+    } else {
+      write32(debug_rom_entry, dmcontrol.hartsel,
+          jal(ZERO, DEBUG_ROM_CODE - (DEBUG_ROM_ENTRY + 4 * dmcontrol.hartsel)));
+    }
 
-    write32(debug_rom_entry, dmcontrol.hartsel,
-        jal(ZERO, DEBUG_ROM_CODE - (DEBUG_ROM_ENTRY + 4 * dmcontrol.hartsel)));
     write32(debug_rom_exception, dmcontrol.hartsel,
         jal(ZERO, (DEBUG_ROM_ENTRY + 4 * dmcontrol.hartsel) - DEBUG_ROM_EXCEPTION));
     abstractcs.busy = true;
@@ -326,7 +354,7 @@ bool debug_module_t::dmi_write(unsigned address, uint32_t value)
     dmdata.write32(4 * (address - DMI_DATA0), value);
     return true;
   } else if (address >= DMI_IBUF0 && address < DMI_IBUF0 + progsize) {
-    ibuf[address - DMI_IBUF0] = value;
+    write32(program_buffer, address - DMI_IBUF0, value);
     return true;
   } else {
     switch (address) {
diff --git a/riscv/debug_module.h b/riscv/debug_module.h
index 15ff66c..fdeaaac 100644
--- a/riscv/debug_module.h
+++ b/riscv/debug_module.h
@@ -101,25 +101,30 @@ class debug_module_t : public abstract_device_t
     bool dmi_write(unsigned address, uint32_t value);
 
   private:
+    static const unsigned progsize = 8;
+
     sim_t *sim;
     // Track which interrupts from module to debugger are set.
     std::set<uint32_t> interrupt;
     // Track which halt notifications from debugger to module are set.
     std::set<uint32_t> halt_notification;
+
     uint8_t debug_rom_entry[DEBUG_ROM_ENTRY_SIZE];
     uint8_t debug_rom_code[DEBUG_ROM_CODE_SIZE];
     uint8_t debug_rom_exception[DEBUG_ROM_EXCEPTION_SIZE];
+    uint8_t program_buffer[progsize * 4];
     bool halted[1024];
     debug_module_data_t dmdata;
+    // Instruction that will be placed at the current hart's ROM entry address
+    // after the current action has completed.
+    uint32_t next_action;
+    bool action_executed;
 
     void write32(uint8_t *rom, unsigned int index, uint32_t value);
     uint32_t read32(uint8_t *rom, unsigned int index);
 
-    static const unsigned progsize = 8;
-
     dmcontrol_t dmcontrol;
     abstractcs_t abstractcs;
-    uint32_t ibuf[progsize];
 
     processor_t *current_proc() const;
     void reset();
-- 
2.30.2