sdcc/device/lib/z80n/divunsigned.s

   1 ;--------------------------------------------------------------------------
   2 ;  divunsigned.s
   3 ;
   4 ;  Copyright (C) 2000-2021, Michael Hope, Philipp Klaus Krause, Marco Bodrato
   5 ;
   6 ;  This library is free software; you can redistribute it and/or modify it
   7 ;  under the terms of the GNU General Public License as published by the
   8 ;  Free Software Foundation; either version 2, or (at your option) any
   9 ;  later version.
  10 ;
  11 ;  This library is distributed in the hope that it will be useful,
  12 ;  but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 ;  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  14 ;  GNU General Public License for more details.
  15 ;
  16 ;  You should have received a copy of the GNU General Public License
  17 ;  along with this library; see the file COPYING. If not, write to the
  18 ;  Free Software Foundation, 51 Franklin Street, Fifth Floor, Boston,
  19 ;   MA 02110-1301, USA.
  20 ;
  21 ;  As a special exception, if you link this library with other files,
  22 ;  some of which are compiled with SDCC, to produce an executable,
  23 ;  this library does not by itself cause the resulting executable to
  24 ;  be covered by the GNU General Public License. This exception does
  25 ;  not however invalidate any other reasons why the executable file
  26 ;   might be covered by the GNU General Public License.
  27 ;--------------------------------------------------------------------------
  28
  29         ;; Originally from GBDK by Pascal Felber.
  30
  31 .area   _CODE
  32
  33 .globl  __divuint
  34 .globl  __divuchar
  35
  36
  37 __divuchar:
  38         ld      e, l
  39         ld      l, a
  40
  41         ;; Fall through
  42 __divu8::
  43         ld      h,#0x00
  44         ld      d,h
  45         ; Fall through to __divu16
  46
  47         ;; unsigned 16-bit division
  48         ;;
  49         ;; Entry conditions
  50         ;;   HL = dividend
  51         ;;   DE = divisor
  52         ;;
  53         ;; Exit conditions
  54         ;;   DE = quotient
  55         ;;   HL = remainder
  56         ;;   carry = 0
  57         ;;   If divisor is 0, quotient is set to "infinity", i.e HL = 0xFFFF.
  58         ;;
  59         ;; Register used: AF,B,DE,HL
  60 __divuint:
  61 __divu16::
  62         ;; Two algorithms: one assumes divisor <2^7, the second
  63         ;; assumes divisor >=2^7; choose the applicable one.
  64         ld      a,e
  65         and     a,#0x80
  66         or      a,d
  67         jr      NZ,.morethan7bits
  68         ;; Both algorithms "rotate" 24 bits (H,L,A) but roles change.
  69
  70         ;; unsigned 16/7-bit division
  71 .atmost7bits:
  72         ld      b,#16           ; bits in dividend and possible quotient
  73         ;; Carry cleared by AND/OR, this "0" bit will pass trough HL.[*]
  74         adc     hl,hl
  75 .dvloop7:
  76         ;; HL holds both dividend and quotient. While we shift a bit from
  77         ;;  MSB of dividend, we shift next bit of quotient in from carry.
  78         ;; A holds remainder.
  79         rla
  80
  81         ;; If remainder is >= divisor, next bit of quotient is 1.  We try
  82         ;;  to compute the difference.
  83         sub     a,e
  84         jr      NC,.nodrop7     ; Jump if remainder is >= dividend
  85         add     a,e             ; Otherwise, restore remainder
  86         ;; The add above sets the carry, because sbc a,e did set it.
  87 .nodrop7:
  88         ccf                     ; Complement borrow so 1 indicates a
  89                                 ;  successful subtraction (this is the
  90                                 ;  next bit of quotient)
  91         adc     hl,hl
  92         djnz    .dvloop7
  93         ;; Carry now contains the same value it contained before
  94         ;; entering .dvloop7[*]: "0" = valid result.
  95         ld      e,a             ; DE = remainder, HL = quotient
  96         ex      de, hl
  97         ret
  98
  99 .morethan7bits:
 100         ld      b,#9            ; at most 9 bits in quotient.
 101         ld      a,l             ; precompute the first 7 shifts, by
 102         ld      l,h             ;  doing 8
 103         ld      h,#0
 104         rr      l               ;  undoing 1
 105 .dvloop:
 106         ;; Shift next bit of quotient into bit 0 of dividend
 107         ;; Shift next MSB of dividend into LSB of remainder
 108         ;; A holds both dividend and quotient. While we shift a bit from
 109         ;;  MSB of dividend, we shift next bit of quotient in from carry
 110         ;; HL holds remainder
 111         adc     hl,hl           ; HL < 2^(7+9), no carry, ever.
 112
 113         ;; If remainder is >= divisor, next bit of quotient is 1. We try
 114         ;;  to compute the difference.
 115         sbc     hl,de
 116         jr      NC,.nodrop      ; Jump if remainder is >= dividend
 117         add     hl,de           ; Otherwise, restore remainder
 118         ;; The add above sets the carry, because sbc hl,de did set it.
 119 .nodrop:
 120         ccf                     ; Complement borrow so 1 indicates a
 121                                 ;  successful subtraction (this is the
 122                                 ;  next bit of quotient)
 123         rla
 124         djnz    .dvloop
 125         ;; Take care of the ninth quotient bit! after the loop B=0.
 126         rl      b               ; BA = quotient
 127         ;; Carry now contains "0" = valid result.
 128         ld      d,b
 129         ld      e,a             ; DE = quotient, HL = remainder
 130         ret
 131