Make division routine only do inc/dec on zero crossing, and other improvements

diff --git a/sm.asm b/sm.asm
index 1889d48..f4f022d 100644 (file)
--- a/sm.asm
+++ b/sm.asm
@@ -850,18 +850,10 @@ div_hl_de:
        ld      b,a
        ld      a,c
        ld      c,b
-       jr      c,2$
-       call    div0
-       jr      c,3$
-1$:    ld      d,c
-       ld      e,a
-       pop     bc
-       ret
-
-2$:    call    div1
+       call    div
        jr      nc,1$
-3$:    add     hl,de
-       ld      d,c
+       add     hl,de
+1$:    ld      d,c
        ld      e,a
        pop     bc
        ret
@@ -888,8 +880,8 @@ div_hl_de:
 
 ; the decrement can also be seen as compensating for the extra add hl,de that
 ; may be needed to make negative remainder positive before return to caller,
-; thus leaving things in a consistent state regardless of which exit occurred,
-; to find out if we need the extra add hl,de we can look for even return byte
+; thus leaving quotient in a consistent state regardless of which exit taken,
+; remainder needs the add hl,de if cf=1 returned (equiv. return byte is even)
 
 ; in the following code each sbc hl,de gets an inc a and each add hl,de gets
 ; a dec a, guaranteeing the integrity of the division, the initial scf/rla is
@@ -898,6 +890,9 @@ div_hl_de:
 ; be another shift between the scf/rla and increment/decrement so that the scf
 ; is implicitly in the 100s place, making the code awkward though it's correct 
 
+; now optimized to only inc/dec a when doing zero-crossing, fix above analysis
+
+div:   jr      c,div1
 div0:
  push af
  ld a,'A
@@ -921,13 +916,12 @@ div0:
        rla
 div00: adc     hl,hl
        sbc     hl,de
-       jr      nc,1$
-       add     a,a
-       inc     a
+       jr      nc,div01
+       dec     a
        jr      div11
-1$:    add     a,a
-       inc     a
-div01: djnz    div00
+div01: add     a,a
+       djnz    div00
+       dec     a
        or      a
  push af
  ld a,'B
@@ -968,18 +962,16 @@ div1:
  call print_word
  ex de,hl
        ld      b,8
-       scf
-       rla
+       add     a,a
 div10: adc     hl,hl
        add     hl,de
-       jr      nc,1$
-       add     a,a
-       dec     a
+       jr      nc,div11
+       inc     a
        jr      div01
-1$:    add     a,a
-       dec     a
-div11: djnz    div10
-       dec     a                       ; compensation
+div11: add     a,a
+       djnz    div10
+       ;inc    a
+       ;dec    a                       ; compensation
        scf
  push af
  ld a,'D