complex nybble CPU

registers 0 to 7 mapped to memory 0 to 7,

opcodes
0        nop
1xy      set *x,y
2xy      set *x,*y
2xy      swap *x,*y
3x       jmp x             jumping to 0-7 jumps to the value of register 0 to 7
4x       jsr x             uses register 7 as stack pointer, and same rule as above
50       rts               return
51x      rts x             return and pop X words
52       skip
53x      ifez              skip if x !=0
54x      ifnz              skip if x ==0
55xy     ifne              skip if x == y
56xy     ifeq              skip if x != y
57xy     ifle              skip if x >  y
58xy     iflt              skip if x >= y
59
5a
5b
5c
5d
5e
5f
6
7
8
9
a
b
c
d
e
f

params
0     constant 0
1     constant 1
2     constant 2
3     constant 3
4     constant 4
5     constant 5
6     constant 6
7     constant 7
8     *(x)
9xy   add x,y
axx   constant (register size)
b0x   negate x
b1x   complement x
b2xy  mul x,y
b3xy  div x,y
b4xy  mod x,y
b5xy  and x,y
b6xy  or x,y
b7xy  xor x,y
b8xy  adc x,y
b9xy  sub x,y
baxy  sbc x,y
bbx   shift left 1
bcx   shift left 2
bdx   shift left 3
bexy  shift X by amount Y
bf    reserved
cx    constant x as 4-bit positive integer
d0x   *(*(x))               register indirect
d1x   *(*(x)++)             push register (the ++ adds register size, probably 2)
d2x   *(--*(x))             pop register (the -- subtracts register size, probably 2)
d3xy  *(*(x)+y)             register indirect plus index
d4xy  *(*(x+y))             ???
d5    *(*(r7)++)            push stack
d6    *(--*(r7))            pop stack
d7
d8
d9
da
db
dc
dd
de
df
e     unused
f     unused