Document multiplication with register allocation

diff --git a/siggen.asm b/siggen.asm
index 351775574b0d53b6c3221bbee6cfb95ecbbacd36..d6b081ec459e1ebe9ec553266423f51459faf34b 100644 (file)
--- a/siggen.asm
+++ b/siggen.asm
@@ -259,6 +259,25 @@ _adc_wait:
        ldi     r20, LOW(STEP_MAX - STEP_MIN)
        ldi     r21, HIGH(STEP_MAX - STEP_MIN)
 
+       ; 16-bit multiplication from 8-bit multiplications
+       ; Like multiplying two two-digit numbers, but each "digit" is now a byte
+       ;
+       ;               r25  r24
+       ;                 A    B  <- value from ADC
+       ;               r21  r20
+       ;            *    C    D  <- scale factor (STEP_MAX - STEP_MIN)
+       ;           ------------
+       ;                r7   r6
+       ;               DBH  DBL
+       ;           r5   r4
+       ;          DAH  DAL
+       ;           r3   r2
+       ;          CBH  CBL
+       ;      r1   r0
+       ;  +  CAH  CAL
+       ;  ---------------------
+       ;      r1   r0   r2   r6  <- 32-bit result
+
        ; Generate partial products
        mul     r20, r24
        movw    r6, r0