76 lines
1.5 KiB
ArmAsm
76 lines
1.5 KiB
ArmAsm
.define .cmu
|
|
.sect .text
|
|
.sect .rom
|
|
.sect .data
|
|
.sect .bss
|
|
.sect .text
|
|
|
|
! parameters:
|
|
! hl : size (#bytes)
|
|
! stack: second operand
|
|
! first operand
|
|
! stack: result (out)
|
|
|
|
|
|
|
|
.cmu:
|
|
! The two operands are compared byte by byte,
|
|
! starting at the highest byte, until
|
|
! they differ.
|
|
pop ix ! return address
|
|
pop hl ! #bytes
|
|
ld b,h ! bc := hl
|
|
ld c,l
|
|
add hl,sp
|
|
dec hl ! pointer to highest byte
|
|
! of second operand
|
|
ld d,h ! de := hl
|
|
ld e,l
|
|
add hl,bc ! pointer to highest byte
|
|
! of first operand
|
|
ld sp,hl ! points to where the
|
|
! result will be stored
|
|
ex de,hl
|
|
! now, de points to highest byte of 1st operand
|
|
! sp ,, ,, ,,
|
|
! hl ,, ,, 2nd ,,
|
|
! bc contains #bytes
|
|
|
|
0:
|
|
! loop, compare the two operands
|
|
! byte by byte.
|
|
ld a,(de)
|
|
xor (hl) ! Avoid overflow during
|
|
! subtraction. If the
|
|
! signbits differ, then
|
|
! the operands differ.
|
|
jp m,2f ! signbits differ
|
|
ld a,(de) ! signbits are equal,
|
|
! so we can savely
|
|
! compare the bytes.
|
|
sub (hl)
|
|
jr nz,1f ! operands are different
|
|
dec de ! the two bytes are the
|
|
! same, try next bytes,
|
|
! if any.
|
|
dec hl ! bump pointers
|
|
dec bc
|
|
ld a,b ! bc = 0 ?
|
|
or c
|
|
jr nz,0b ! no, try next bytes
|
|
! yes, then the two operands are equal.
|
|
! Note that a=0 now.
|
|
1:
|
|
ld h,a ! hl := result
|
|
ld l,a
|
|
jr 3f
|
|
2:
|
|
! the signbits differ
|
|
ld h,(hl) ! hl := positive if
|
|
! signbit of current
|
|
! byte of 2nd operand
|
|
! is "0", else negative
|
|
ld l,1 ! just in case (hl)=0
|
|
3:
|
|
ex (sp),hl ! sp was set above
|
|
jp (ix) ! return
|