git.sur5r.net Git - cc65/blob - libsrc/specialmath/mul40.s

   1 ; mul40.s
   2 ;
   3 ; This file is part of
   4 ; cc65 - a freeware C compiler for 6502 based systems
   5 ;
   6 ; https://github.com/cc65/cc65
   7 ;
   8 ; See "LICENSE" file for legal information.
   9 ;
  10 ;
  11 ; unsigned int __fastcall__ mul40(unsigned char value);
  12 ;
  13 ; REMARKS: Function is defined to return with carry-flag cleared
  14
  15
  16         .importzp       tmp4
  17         .export         _mul40
  18
  19 .proc   _mul40                  ; = 33 bytes, 48/53 cycles
  20
  21         sta     tmp4            ; remember value for later addition...
  22         ldx     #0              ; clear high-byte
  23         asl     a               ; * 2
  24         bcc     mul4            ; high-byte affected?
  25         ldx     #2              ; this will be the 1st high-bit soon...
  26
  27 mul4:   asl     a               ; * 4
  28         bcc     mul5            ; high-byte affected?
  29         inx                     ; => yes, apply to 0 high-bit
  30         clc                     ; prepare addition
  31
  32 mul5:   adc     tmp4            ; * 5
  33         bcc     mul10           ; high-byte affected?
  34         inx                     ; yes, correct...
  35
  36 mul10:  stx     tmp4            ; continue with classic shifting...
  37
  38         asl     a               ; * 10
  39         rol     tmp4
  40
  41         asl     a               ; * 20
  42         rol     tmp4
  43
  44         asl     a               ; * 40
  45         rol     tmp4
  46
  47         ldx     tmp4            ; deliver high-byte in X
  48         rts
  49
  50 .endproc