RevertMem
Synopsis
RevertMem(start, end, type)
A0 A1 D0:8C Prototype
void RevertMem(APTR start, APTR end, ULONG type);Description
This function reverses the contents of a memory area defined by start and end. It reverses the sequence of bytes, words, or longwords within the specified region. The first byte/word/longword becomes the last, the second becomes the second-to-last, and so on.
The reversal granularity is controlled by the type parameter, which specifies the number of bits to operate on (8, 16, or 32).
Inputs
- start — Start address of the memory area to reverse.
- end — End address of the memory area to reverse. This can be calculated by adding the number of bytes to operate on to
start, regardless of whether the reversal is performed on bytes, words, or longwords. - type — Specifies the number of bits (8, 16, or 32) to operate on. The accepted values are:
TYPEF_BYTE— operates on bytesTYPEF_WORD— operates on wordsTYPEF_LONG— operates on longwords
Result
None.
Example
;
; Reverse 2 words at address:
;
; test: dc.b "ABCD"
;
move.l assemblybase,a6
lea test,a0
lea 4(a0),a1
moveq #TYPEF_WORD,d0
JSR _LVORevertMem(a6)
;
; After the call to RevertMem() you will have:
;
; test: dc.b "CDAB"
;Notes
As shown in the example, the end address is easily calculated starting from the start address. Keep in mind that the difference between end and start must always equal the number of bytes to operate on.
See Also
None.
Implementation
The original 68020 assembly implementation:
; (V41.1) - 21-Oct-1994 --- RevertMem()
_LVORevertMem
move.l a1,d1 ; D1 = number of bytes to reverse
sub.l a0,d1
bhi.s RVM_CON
move.l (sp)+,d2
rts
RVM_CON move.l d2,-(sp)
asr.w d0,d1
asr.w #1,d1
subq.w #1,d1
subq.w #1,d0
bmi.s RVM_BYT
beq.s RVM_WOR
RVM_LON move.l -(a1),d2
move.l (a0)+,(a1)
move.l d2,-4(a0)
dbf d1,RVM_LON
move.l (sp)+,d2
rts
RVM_WOR move.w -(a1),d2
move.w (a0)+,(a1)
move.w d2,-2(a0)
dbf d1,RVM_WOR
move.l (sp)+,d2
rts
RVM_BYT move.b -(a1),d2
move.b (a0)+,(a1)
move.b d2,-1(a0)
dbf d1,RVM_BYT
move.l (sp)+,d2
rtsLast updated on