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48 lines
1.3 KiB
ArmAsm
48 lines
1.3 KiB
ArmAsm
; mul20.s
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;
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; This file is part of
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; cc65 - a freeware C compiler for 6502 based systems
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;
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; https://cc65.github.io
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;
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; See "LICENSE" file for legal information.
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;
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;
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; unsigned int __fastcall__ mul20(unsigned char value);
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;
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; REMARKS: Function is defined to return with carry-flag cleared
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.importzp tmp4
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.export _mul20
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.proc _mul20 ; = 30 bytes, 41/46 cycles
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sta tmp4 ; remember value for later addition...
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ldx #0 ; clear high-byte
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asl a ; * 2
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bcc mul4 ; high-byte affected?
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ldx #2 ; this will be the 1st high-bit soon...
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mul4: asl a ; * 4
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bcc mul5 ; high-byte affected?
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inx ; => yes, apply to 0 high-bit
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clc ; prepare addition
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mul5: adc tmp4 ; * 5
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bcc mul10 ; high-byte affected?
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inx ; yes, correct...
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mul10: stx tmp4 ; continue with classic shifting...
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asl a ; * 10
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rol tmp4
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asl a ; * 20
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rol tmp4
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ldx tmp4 ; deliver high-byte in X
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rts
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.endproc
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