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