mirror of https://github.com/rkujawa/rk65c02.git
51 lines
934 B
ArmAsm
51 lines
934 B
ArmAsm
.org 0xC000
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start: jsr mul_8bit_to_8bits
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stp
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; mul_8bit_to_8bits
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; Multiplies 2 numbers (passed on stack) and returns result on stack.
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; General 8bit * 8bit = 8bit multiply
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; by White Flame 20030207
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; adapted as rk65c02 example by rkujawa
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; Instead of using a bit counter, this routine early-exits when num2 reaches
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; zero, thus saving iterations.
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; .X and .Y are preserved
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; num1 and num2 get clobbered
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.set num1,0x10
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.set num2,0x11
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.set retl,0x12
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.set reth,0x13
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mul_8bit_to_8bits:
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pla ; pull return address
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sta retl
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pla
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sta reth
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pla ; pull num1 from stack
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sta num1
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pla ; pull num2 from stack
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sta num2
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lda #0x00
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beq enterl
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doAdd: clc
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adc num1
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loop: asl num1
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; For an accumulating multiply (A = A + num1 * num2),
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; set up num1 and num2, then enter here.
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enterl: lsr num2
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bcs doAdd
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bne loop
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end:
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pha ; save result to stack
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lda reth ; restore return address
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pha
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lda retl
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pha
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rts ; return from function
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