Z80 Divide by 10

[Brent Hilpert]

dwight elvey wrote:
>  I think my routine may be extended a few bytes by changing the
> tweak value. It has greater effect for small values but could get one
> or two numbers increased if optimized.
>  Such tweaks are there to make up for the way we truncate instead
> of rounding. It means the value is always too small after a large number
> of operation.

Dwight, if my test-code version of your routine is correct, the constant
correction can be reduced to 1 from 16 and the routine then fails at 1280
instead of 1210. The addition can now be changed to an increment and some other
register optimisation done to take another two bytes and some cycles off (no
need to use A any longer, quotient is in C):

------------------------------
Segment: MAIN
  base=$0000 end=$001D bytes=30

           * 0-799 divided by 10
           * implements: quo = ( 51*x + (51*x/256+1) ) / 512
           *             rem = x - (q*4+q)*2
           * hl = dividend
           * Returns:
           * c = quotient
           * l = remainder
                  
                      machine z80
  
0000 54    Div10fast  LD      D,H       de = hl
0001 5D               LD      E,L
  
0002 29               ADD     HL,HL
0003 29               ADD     HL,HL
0004 19               ADD     HL,DE     x * 5
0005 29               ADD     HL,HL     x * 10
0006 44               LD      B,H       bc = 10x
0007 4D               LD      C,L
0008 29               ADD     HL,HL
0009 29               ADD     HL,HL
000A 09               ADD     HL,BC     x * 50
000B 19               ADD     HL,DE     hl = x*50 + x
  
000C 06 00            LD      B,0       bc = hl/256
000E 4C               LD      C,H
000F 03               INC     BC        + 1
0010 09               ADD     HL,BC     ADD little more accuracy
  
0011 4C               LD      C,H       c = hl / 256 / 2
0012 CB 19            RR      C         quo = c, carry cleared in ADD
  
0014 60               LD      H,B       hl = quo
0015 69               LD      L,C
0016 29               ADD     HL,HL     quo*10 to calc RemainDEr
0017 29               ADD     HL,HL
0018 09               ADD     HL,BC     
0019 29               ADD     HL,HL     
001A EB               EX      DE,HL
001B ED 52            SBC     HL,DE     rem = l
  
001D C9               RET
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