avr: add 8 bit multiplication.

avr/common/0BUILD

@@ -21,6 +21,7 @@
     m_ringbuffer.asm
     m_ringbuffer_y.asm
     multiply.asm
+    multiply8.asm
     random.asm
     ressource.asm
     ringbuffer.asm

avr/common/multiply8.asm

@@ -0,0 +1,55 @@
+; ***************************************************************************
+; copyright   : (C) 2026 by Martin Preuss
+; email       : martin@libchipcard.de
+;
+; ***************************************************************************
+; * This file is part of the project "AqHome".                              *
+; * Please see toplevel file COPYING of that project for license details.   *
+; ***************************************************************************
+
+#ifndef COMMON_MULTIPLY8_H
+#define COMMON_MULTIPLY8_H
+
+
+; ---------------------------------------------------------------------------
+; @routine Utils_Mulu8x8_16
+;
+; Multiplies two unsigned 8 bit values resulting in one 16 bit value.
+; This is a rather simple but reasonable fast routine working just like you would
+; when doing multiplications with pen and paper.
+;
+; R17:R16 = R18 * R19
+;
+; TODO: adapt for signed values
+;
+; @param  R18     8 bit value A
+; @param  R19     8 bit value B
+; @return R17:R16 16 bit result
+; @clobbers R19, R20, R21
+
+Utils_Mulu8x8_16:
+  clr r17
+  clr r16
+  clr r20
+  ldi r21, 8                     ; 8 bit multiplicator
+Utils_Mulu8x8_16_loop:
+    lsr r19
+    brcc Utils_Mulu8x8_16_noadd ; current digit in B is 0, don't add shifted A to result
+    add r16, r18
+    adc r17, r20
+Utils_Mulu8x8_16_noadd:
+    dec r21
+    breq Utils_Mulu8x8_16_done
+    lsl r18
+  rjmp Utils_Mulu8x8_16_loop
+Utils_Mulu8x8_16_done:
+  clc
+  ret
+; @end
+
+
+
+
+
+#endif ; COMMON_MULTIPLY8_H
+