130 lines
2.4 KiB
ArmAsm
130 lines
2.4 KiB
ArmAsm
; http://www.llx.com/~nparker/a2/mult.html
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; MULTIPLY NUM1H:NUM1L * NUM2H:NUM2L
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; NUM2 is zeroed out
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; result is in RESULT3:RESULT2:RESULT1:RESULT0
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;NUM1L: .byte 0
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;NUM1H: .byte 0
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;NUM2L: .byte 0
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;NUM2H: .byte 0
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;RESULT: .byte 0,0,0,0
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;NEGATE: .byte 0
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; If we have 2k to spare we should check out
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; http://codebase64.org/doku.php?id=base:seriously_fast_multiplication
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multiply:
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lda #$0 ; 2
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sta NEGATE ; 3
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; Handle Signed
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lda NUM1H ; 3
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bpl check_num2 ; 2nt/3
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;==============
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; 10
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inc NEGATE ; 3
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clc ; 2s-complement NUM1H/NUM1L ; 2
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lda NUM1L ; 3
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eor #$ff ; 2
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adc #$1 ; 2
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sta NUM1L ; 3
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lda NUM1H ; 3
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eor #$ff ; 2
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adc #$0 ; 2
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sta NUM1H ; 3
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;===========
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; 25
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check_num2:
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lda NUM2H ; 3
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bpl unsigned_multiply ; 2nt/3
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;==============
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; 6
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inc NEGATE ; 3
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clc ; 2
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lda NUM2L ; 3
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eor #$ff ; 2
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adc #$1 ; 2
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sta NUM2L ; 3
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lda NUM2H ; 3
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eor #$ff ; 2
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adc #$0 ; 2
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sta NUM2H ; 3
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;=============
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; 25
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unsigned_multiply:
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lda #0 ; Initialize RESULT to 0 ; 2
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sta RESULT+2 ; 3
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ldx #16 ; 16x16 multiply ; 2
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;============
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; 7
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multiply_mainloop:
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lsr NUM2H ; Shift right 16-bit NUM2 ; 5
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ror NUM2L ; low bit goes into carry ; 5
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bcc shift_output ; 0 or 1? ; 2nt/3
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;============
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; 13
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tay ; If 1, add NUM1 (hi byte RESULT in A) ; 2
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clc ; 2
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lda NUM1L ; 3
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adc RESULT+2 ; 3
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sta RESULT+2 ; 3
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tya ; 2
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adc NUM1H ; 3
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;============
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; 18
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shift_output:
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ror A ; "Stairstep" shift ; 2
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ror RESULT+2 ; 5
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ror RESULT+1 ; 5
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ror RESULT ; 5
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dex ; 2
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bne multiply_mainloop ; 2nt/3
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;=============
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; 22
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sta RESULT+3 ; 3
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;; Negate if necessary
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lda NEGATE ; 3
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and #$1 ; 2
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beq positive ; 2nt/3
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;==============
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; 11
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clc ; 2
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lda RESULT+0 ; 3
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eor #$ff ; 2
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adc #$1 ; 2
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sta RESULT+0 ; 3
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lda RESULT+1 ; 3
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eor #$ff ; 2
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adc #$0 ; 2
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sta RESULT+1 ; 3
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lda RESULT+2 ; 3
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eor #$ff ; 2
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adc #$0 ; 2
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sta RESULT+2 ; 3
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lda RESULT+3 ; 3
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eor #$ff ; 2
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adc #$0 ; 2
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sta RESULT+3 ; 3
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;===========
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; 42
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positive:
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rts ; 6
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