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cc65/libsrc/common/mul20.s
2020-02-03 20:45:09 +01:00

48 lines
1.3 KiB
ArmAsm

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