Reversing the bytes in a word: (i.e. changing endianness)

On comp.sys.acorn nbannon@tartarus.uwa.edu.au (Nick Bannon) gave a small code fragment to print decimal numbers from ARM code - useful for debugging, and olly@mantis.co.uk (Olly Betts) made a small bug fix. The final version is included below: 
    .printbin			; R0=num
    STMFD R13!,{R0-R2,R14}
    SUB R13,R13,#36
    MOV R1,R13
    MOV R2,#36
    SWI XOS_ConvertBinary4	; Convert to 32 char string

    ;._loop			; Strip leading 0's
    ;LDRB R1,[R0],#1		; Uncomment to execute
    ;CMP R1,#"0"
    ;BEQ _loop
    ;SBC R0,R0,#1		; subtracts 1 if R1>='0', 2 if R1<'0'

    SWI XOS_Write0
    ADD R13,R13,#36
    SWI XOS_NewLine
    LDMFD R13!,{R0-R2,PC}^
Robin.Watts@prg.ox.ac.uk