from nmutil.latch import SRLatch, latchregister
+""" Computation Unit (aka "ALU Manager").
+
+ This module runs a "revolving door" set of three latches, based on
+ * Issue
+ * Go_Read
+ * Go_Write
+ where one of them cannot be set on any given cycle.
+ (Note however that opc_l has been inverted (and qn used), due to SRLatch
+ default reset state being "0" rather than "1")
+
+ * When issue is first raised, a busy signal is sent out.
+ The src1 and src2 registers and the operand can be latched in
+ at this point
+
+ * Read request is set, which is acknowledged through the Scoreboard
+ to the priority picker, which generates (one and only one) Go_Read
+ at a time. One of those will (eventually) be this Computation Unit.
+
+ * Once Go_Read is set, the src1/src2/operand latch door shuts (locking
+ src1/src2/operand in place), and the ALU is told to proceed.
+
+ * As this is currently a "demo" unit, a countdown timer is activated
+ to simulate an ALU "pipeline", which activates "write request release",
+ and the ALU's output is captured into a temporary register.
+
+ * Write request release will go through a similar process as Read request,
+ resulting (eventually) in Go_Write being asserted.
+
+ * When Go_Write is asserted, two things happen: (1) the data in the temp
+ register is placed combinatorially onto the output, and (2) the
+ req_l latch is cleared, busy is dropped, and the Comp Unit is back
+ through its revolving door to do another task.
+"""
class ComputationUnitNoDelay(Elaboratable):
def __init__(self, rwid, opwid, alu):
self.go_rd_i = Signal(reset_less=True) # go read in
self.go_wr_i = Signal(reset_less=True) # go write in
self.issue_i = Signal(reset_less=True) # fn issue in
+ self.shadown_i = Signal(reset=1) # shadow function, defaults to ON
+ self.go_die_i = Signal() # go die (reset)
self.oper_i = Signal(opwid, reset_less=True) # opcode in
+ self.imm_i = Signal(rwid, reset_less=True) # immediate in
self.src1_i = Signal(rwid, reset_less=True) # oper1 in
self.src2_i = Signal(rwid, reset_less=True) # oper2 in
m.submodules.opc_l = opc_l = SRLatch(sync=False)
m.submodules.req_l = req_l = SRLatch(sync=False)
+ # shadow/go_die
+ reset_w = Signal(reset_less=True)
+ reset_r = Signal(reset_less=True)
+ m.d.comb += reset_w.eq(self.go_wr_i | self.go_die_i)
+ m.d.comb += reset_r.eq(self.go_rd_i | self.go_die_i)
+
# This is fascinating and very important to observe that this
# is in effect a "3-way revolving door". At no time may all 3
# latches be set at the same time.
# opcode latch (not using go_rd_i) - inverted so that busy resets to 0
m.d.sync += opc_l.s.eq(self.issue_i) # XXX NOTE: INVERTED FROM book!
- m.d.sync += opc_l.r.eq(self.go_wr_i) # XXX NOTE: INVERTED FROM book!
+ m.d.sync += opc_l.r.eq(reset_w) # XXX NOTE: INVERTED FROM book!
# src operand latch (not using go_wr_i)
m.d.sync += src_l.s.eq(self.issue_i)
- m.d.sync += src_l.r.eq(self.go_rd_i)
+ m.d.sync += src_l.r.eq(reset_r)
# dest operand latch (not using issue_i)
m.d.sync += req_l.s.eq(self.go_rd_i)
- m.d.sync += req_l.r.eq(self.go_wr_i)
+ m.d.sync += req_l.r.eq(reset_w)
# XXX
# XXX NOTE: sync on req_rel_o and data_o due to simulation lock-up
# XXX
# outputs
- m.d.comb += self.busy_o.eq(opc_l.q) # busy out
- m.d.comb += self.rd_rel_o.eq(src_l.q & opc_l.q) # src1/src2 req rel
-
- with m.If(req_l.qn & opc_l.q & (self.counter == 0)):
- with m.If(self.oper_i == 2): # MUL, to take 5 instructions
+ busy_o = self.busy_o
+ m.d.comb += busy_o.eq(opc_l.q) # busy out
+ m.d.comb += self.rd_rel_o.eq(src_l.q & busy_o) # src1/src2 req rel
+
+ # the counter is just for demo purposes, to get the ALUs of different
+ # types to take arbitrary completion times
+ with m.If(opc_l.qn):
+ m.d.sync += self.counter.eq(0)
+ with m.If(req_l.qn & busy_o & (self.counter == 0)):
+ with m.If(self.alu.op == 2): # MUL, to take 5 instructions
m.d.sync += self.counter.eq(5)
- with m.Elif(self.oper_i == 3): # SHIFT to take 7
+ with m.Elif(self.alu.op == 3): # SHIFT to take 7
m.d.sync += self.counter.eq(7)
+ with m.Elif(self.alu.op >= 4): # Branches take 6 (to test shadow)
+ m.d.sync += self.counter.eq(6)
with m.Else(): # ADD/SUB to take 2
m.d.sync += self.counter.eq(2)
- with m.If(self.counter > 0):
+ with m.If(self.counter > 1):
m.d.sync += self.counter.eq(self.counter - 1)
- with m.If((self.counter == 1) | (self.counter == 0)):
- m.d.comb += self.req_rel_o.eq(req_l.q & opc_l.q) # req release out
+ with m.If(self.counter == 1):
+ # write req release out. waits until shadow is dropped.
+ m.d.comb += self.req_rel_o.eq(req_l.q & busy_o & self.shadown_i)
# create a latch/register for src1/src2
latchregister(m, self.src1_i, self.alu.a, src_l.q)
# m.d.comb += self.alu.op.eq(self.oper_i)
# create a latch/register for the operand
- latchregister(m, self.oper_i, self.alu.op, src_l.q)
+ latchregister(m, self.oper_i, self.alu.op, self.issue_i)
# and one for the output from the ALU
- data_o = Signal(self.rwid, reset_less=True) # Dest register
- latchregister(m, self.alu.o, data_o, req_l.q)
+ data_r = Signal(self.rwid, reset_less=True) # Dest register
+ latchregister(m, self.alu.o, data_r, req_l.q)
with m.If(self.go_wr_i):
- m.d.comb += self.data_o.eq(data_o)
+ m.d.comb += self.data_o.eq(data_r)
return m
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