pdp-1

http://research.microsoft.com/en-us/um/people/gbell/Digital/PDP%201%20Manual%201961.pdf

accumulator
in/out register

opcode format:
o oooo i aaaa aaaa aaaa - opcode, indirect, address

AC = accumulator
Y = address pointed to by address part of opcode
y = address part of opcode as a constant
M[n] = memory at n
IO = In-Out register

add Y: AC = AC + Y (1's complement add)
sub Y: AC = AC - Y (1's complement subtract)
mus Y: multiply step, weird stuff
dis Y: divide step, weird stuff
idx Y: Y++; AC = Y
isp Y: Y++; AC = Y; if Y > 0, skip next
and Y: AC = AC & Y
xor Y: AC = AC ^ Y
ior Y: AC = AC | Y
lac Y: AC = Y
dac Y: Y = AC
dap Y: address bits in Y are replaced with corresponding bits in AC
dip Y: opcode bits in Y are replaced with corresponding bits in AC
lio Y: IO = Y
dio Y: Y = IO
dzm Y: Y = 0
xct Y: run opcode pointed to by y, don't change PC
jmp Y: PC = y
jsp Y: AC = PC; PC = y
cal Y: M[64] = AC; AC = PC; PC = M[65]
jda Y: Y = AC; AC = PC; PC = y+1
sad Y: skip if AC != Y
sas Y: skip if AC == Y
law Y: if indirect = 0, A = y; if indirect = 1, A = ~y
rar Y: rotate AC right N positions, where N is the number of 1s in y
ral Y: same but left
sar Y: shift AC right N positions, where N is the number of 1s in y
sal Y: same but left
rir Y: rar but IO register instead of AC
ril Y: ral but IO register instead of AC
sir Y: sar but IO register instead of AC
sil Y: sal but IO register instead of AC
rcr Y: rotate AC:IO N bits right
rcl Y: rotate AC:IO N bits left
scr Y: shift AC:IO N bits right
scl Y: shift AC:IO N bits left
sza  : skip if AC == 0
spa  : skip if AC positive
sma  : skip if AC negative
szo  : skip if overflow zero
spi  : skip if IO positive
szs  : skip if switch (one out of 6, or all 6) is zero
szf  : skip if program flag (out of 8) zero

(these opcodes can be combined)
cli  : IO = 0
lat  : A |= test word switches
cma  : A = -A
hlt  : stop computer
cla  : clear accumulator
clf  : clear program flag (out of 8)
stf  : set program flag (out of 8)
nop  : do nothing

iot  : I/O thing

multiplication and division as an option