Reference Guide

132 Chapter 7
Programming Examples
2. Copying a String
2. Copying a String
This example contains a section of assembly code that moves a byte
string of arbitrary length to an arbitrary byte address.
; The routine reflect copies bytes from the source location
; to the destination location.
;
; The first parameter is the source address and the second
; parameter is the destination address.
;
; The third parameter is the number of bytes to copy.
;
; For performance, larger chunks of bytes are handled differently.
;
.CODE
.EXPORT reflect,ENTRY
reflect
.PROC
.CALLINFO ENTRY_GR=6,SAVE_RP
.ENTER
COMB,=,N %arg2,%r0,fallout ; done, count is zero
COMB,<,N %arg2,%r0,choke ; caller error, neg count
OR %arg0,%arg1,%r6 ; source and dest
OR %r6,%arg2,%r6 ; count
EXTRU,= %r6,31,2,%r0 ; 2 low order bits = 0?
B,N onebyte ; yes, skip this branch
ADDIBT,<,N -16,%arg2,chekchunk ; no, skip chunkify if count<0
chunkify
LDWM 16(%arg0),%r6 ; word 1- > temp1
; point ahead 4 words in source
LDW -12(%arg0),%r5 ; place mark 3 wds back- >temp2
LDW -8(%arg0),%r4 ; place mark 2 wds back- >temp3
LDW -4(%arg0),%r3 ; place mark 1 wds back- >temp4
STW %r5,4(%arg1) ; dest wd 2 <-temp2
STWM %r6,16(%arg1) ; dest wd 1 <-temp1
; point ahead 4 words in dest
STW %r4,-8(%arg1) ; dest wd 3 <-temp3
ADDIBF,< -16,%arg2,chunkify ; loop if count > 0
STW %r3,-4(%arg1) ; dest wd 2 <-temp1
chekchunk
ADDIBT,< 12,%arg2,exeunt ; go if count < -12
COPY %r0,%ret0 ; clear rtnval
subchunk
LDWS,MA 4(%arg0),%r6 ; word- >temp1
; point ahead 4 bytes in source
ADDIBF,< -4,%arg2,subchunk ; go if count<4
STWS,MA %r6,4(%arg1) ; dest< -temp1
; point ahead 4 bytes in dest