Developing Software in Assembly Language
6812 Assembly Language Shift Examples
By Jonathan W. Valvano

     This article, which discusses assembly language programming, accompanies the book Embedded Microcomputer Systems: Real Time Interfacing published by Brooks-Cole 1999. This document has four overall parts
     Overview
     Syntax (fields, pseudo ops)
     Local variables
     Examples

Shift Examples on the 6812
      8-bit shift left
      8-bit unsigned shift right
      8-bit signed shift right
      32-bit shift left
      32-bit unsigned shift right
      32-bit signed shift right

Other Examples
      Arithmetic Examples on the 6812
      Stack Usage and Interrupts on the 6812
      Interpreters on the 6812
      FIFO Queue Examples on the 6812
      Control Structure Examples on the 6812 

--------------------------------------------------------------------------------------
8-bit arithmetic left shift, unsigned or signed

      org  $0800
data  rmb  1
      org  $F000
; make data=6*data (signed 8 bit)
; V=1 if overflow
Mul6  ldaa data
      tab      ;save a copy
      asla     ;times 2
      bvs  err
      aba      ;times 3
      bvs  err
      asla     ;times 6
err   staa data
      rts


      org  $0800
data  rmb  1
      org  $F000
; make data=4*data (signed 8 bit)
; V=1 if overflow
Mul4  ldab data
      aslb      ;times 2
      bvs  err
      aslb      ;times 4
err   stab data
      rts

--------------------------------------------------------------------------------------
8-bit arithmetic right shift, signed
      org  $0800
data  rmb  1
      org  $F000
; make data=data/4 (signed 8-bit)
Div4  asr  data   ; divide by 2
      asr  data   ; divide by 4
      rts

--------------------------------------------------------------------------------------
8-bit logic right shift, unsigned
      org  $0800
data  rmb  1
      org  $F000
; make data=data/4 (unsigned 8-bit)
Div4  lsr  data   ; divide by 2
      lsr  data   ; divide by 4
      rts

--------------------------------------------------------------------------------------
32-bit shift left N=N<<1 (unsigned and signed) on the 6812

N      ds   4     ; 32-bit number
asl32  asl  N+3   ; start with least significant byte
       rol  N+2   ; next byte
       rol  N+1   ; next byte
       rol  N     ; last byte
; C bit set if unsigned overflow
; V bit set if signed overflow
      rts

--------------------------------------------------------------------------------------
32-bit signed shift right N=N>>1 on the 6812

N     ds   4     ; 32-bit number
asr32 asr  N     ; start with most significant byte
      ror  N+1   ; next byte
      ror  N+2   ; next byte
      ror  N+3   ; last byte
; C bit set if you should round up
      rts

;with rounding
asr32r asr   N     ; start with most significant byte
       ror   N+1   ; next byte
       ror   N+2   ; next byte
       ror   N+3   ; last byte
; C bit set if you should round up
       bcc   Done
       inc   N+3   ; round up
       bcc   Done
       inc   N+2   ; round up
       bcc   Done
       inc   N+1   ; round up
       bcc   Done
       inc   N     ; round up
Done   rts

--------------------------------------------------------------------------------------
32-bit unsigned shift right N=N>>1 on the 6812

N     ds    4     ; 32-bit number
lsr32 lsr   N     ; start with most significant byte
      ror   N+1   ; next byte
      ror   N+2   ; next byte
      ror   N+3   ; last byte
; C bit set if you should round up
      rts

;with rounding
lsr32r lsr   N     ; start with most significant byte
       ror   N+1   ; next byte
       ror   N+2   ; next byte
       ror   N+3   ; last byte
; C bit set if you should round up
       bcc   Done
       inc   N+3   ; round up
       bcc   Done
       inc   N+2   ; round up
       bcc   Done
       inc   N+1   ; round up
       bcc   Done
       inc   N     ; round up
Done   rts

This document has four overall parts
     Overview
     Syntax (fields, pseudo ops)
     Local variables
     Examples