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SIMH allows -(PC) addressing mode, therefore, so do we

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This commit is contained in:
Neil Webber 2024-05-08 20:37:57 -05:00
parent d1e040dce8
commit f278e18b89
1 changed files with 10 additions and 7 deletions
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17
machine.py
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@ -468,15 +468,18 @@ class PDP11:
autocrement = 2 # regardless of opsize used above
extendedb6 = None # force update below
# both autodecrement addrmode, PC - NOPE.
case 0, 0o40 | 0o50, 7:
# ... did the pdp11 fault on this?
raise PDPTraps.ReservedInstruction
# both autodecrement addrmodes, not PC
# both autodecrement addrmodes
# note that bytes and -(SP) still decrement by 2
# notes for -(PC):
# * SIMH allows it (causes tight loop of course)
# * Some DEC manuals say "DO NOT USE THIS FORM"
# * Some DEC manuals tell you that, like SP, the
# -(PC) autodecrement will be 2, even for bytes.
# No DEC manuals (of those surveyed anyway) specify
# that -(PC) will trap in some way. So, it's allowed here.
#
case 0, 0o40 | 0o50 as addrmode, Rn:
if Rn == self.SP:
if Rn == self.SP or Rn == self.PC:
autocrement = -2
elif addrmode == 0o50:
autocrement = -2