56 lines
1.4 KiB
NASM
56 lines
1.4 KiB
NASM
; ***************************************************************************
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; copyright : (C) 2026 by Martin Preuss
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; email : martin@libchipcard.de
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;
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; ***************************************************************************
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; * This file is part of the project "AqHome". *
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; * Please see toplevel file COPYING of that project for license details. *
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; ***************************************************************************
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#ifndef COMMON_MULTIPLY8_H
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#define COMMON_MULTIPLY8_H
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; ---------------------------------------------------------------------------
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; @routine Utils_Mulu8x8_16
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;
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; Multiplies two unsigned 8 bit values resulting in one 16 bit value.
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; This is a rather simple but reasonable fast routine working just like you would
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; when doing multiplications with pen and paper.
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;
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; R17:R16 = R18 * R19
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;
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; TODO: adapt for signed values
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;
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; @param R18 8 bit value A
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; @param R19 8 bit value B
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; @return R17:R16 16 bit result
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; @clobbers R19, R20, R21
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Utils_Mulu8x8_16:
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clr r17
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clr r16
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clr r20
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ldi r21, 8 ; 8 bit multiplicator
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Utils_Mulu8x8_16_loop:
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lsr r19
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brcc Utils_Mulu8x8_16_noadd ; current digit in B is 0, don't add shifted A to result
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add r16, r18
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adc r17, r20
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Utils_Mulu8x8_16_noadd:
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dec r21
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breq Utils_Mulu8x8_16_done
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lsl r18
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rjmp Utils_Mulu8x8_16_loop
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Utils_Mulu8x8_16_done:
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clc
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ret
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; @end
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#endif ; COMMON_MULTIPLY8_H
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