M68k Big Endian to Little Endian

Question

I'm trying to write a binary file using vasm68k_mot (Motorola 68k) writing File-Size and File-Offset everything goes well, except that M68K is Big-Endian, there's a way I can change to Little-Endian in a simple way?

Let's say that the FileZ.bin has a size of 30Kb, this in hex should be 0x7530 in my binary file it writes in hex value "00 00 75 30", but I need to mirror that value to "30 75 00 00" The same applies for the file offset address.

I'm sorry if this is confusing, I'm not a programmer nor expert in ASM.

My code is like this:

dc.l    0,(FILEY_END-FILEY_START)               
dc.l    FILEZ_START,(FILEZ_END-FILEZ_START)
dc.l    FILEX_START,(FILEX_END-FILEX_START)  
cnop    0,2048



FILEZ_START
    incbin C:\FileZ.BIN
FILEZ_END
    cnop    0,2048

FILEX_START
FILEY_START
    incbin C:\FileY.BIN
FILEY_END
    cnop    0,2048
    incbin C:\FileX.bin
FILEX_END

EDIT: The documentation provided by @AlexHajnal is the same i'm using, thank you! I also will link it here: http://sun.hasenbraten.de/vasm/release/vasm.pdf

EDIT: All I need to is SWAP those "dc.l" values, they are 4 bytes long, I tried writing

ROL.W   #8,D0
SWAP    D0
ROL.W   #8,D0

but i got some random numbers, it may be related to vasmm68k_mot, but i really don't know....

Answer 1

My original take on the question:

For longs (32-bit), this is answered here. For shorts (16-bit) just use ROL.W #8, D0.

---

If @thebusybee's interpretation of the question is correct (which appears to be the case) then the answer would appear to be "No" (when targeting MC68000). From page 6 of the vasm manual:

... a constant is built according to the endianess of the target.

However, if you change the target to something little-endian (e.g. x86 instead of 68000) then computed values will be stored as little-endian.

To use a different target you'll need to change the syntax of your assembler source file. For example, the x86 equivalent of the source you posted (as of 2021-05-19 21:45Z) is:

.long 0,FILEY_END-FILEY_START
.long FILEZ_START,FILEZ_END-FILEZ_START
.long FILEX_START,FILEX_END-FILEX_START  
.balign    2048



FILEZ_START:
    .incbin C:\FileZ.BIN
FILEZ_END:
    .balign    2048

FILEX_START:
FILEY_START:
    .incbin C:\FileY.BIN
FILEY_END:
    .balign    2048
    .incbin C:\FileX.BIN
FILEX_END:

The above should be placed in a file named file.s

To create your output file (to be named file.bin) run:

vasmx86_std -Fbin -o file.bin file.s

You may need to tweak the file names/paths above (I tested on Linux then added the C:\ prefixes to what I posted above).

Answer 2

This is just an alternative for the poor lost souls coming here for help. ;-)

Instead of code to be run by the processor, this solution generates a 32-bit value in little endian right in place.

The assembler has macro capabilities, and a macro can be used:

• dc_l_le is the name of the macro. Look at it as a new pseudo instruction. It means "define constant, long, little endian".
• The macro handles different number of expressions, tested with 2, but I'm sure there is a limit.
• The number of expressions is obtained by \# and used for a repetition loop.
• The n-th expression can be obtained by \+. This special value inserts one expression after the other. On each repetition the local symbol .value is set to the expression.
• Shifting and masking separates the bytes.

dc_l_le macro
        rept    \#
.value  set     \+
        dc.b    (.value>>0)&$FF
        dc.b    (.value>>8)&$FF
        dc.b    (.value>>16)&$FF
        dc.b    (.value>>24)&$FF
        endr
        endm

    org $12345678

    dc_l_le 0,(FILEY_END-FILEY_START)               
    dc_l_le FILEZ_START,(FILEZ_END-FILEZ_START)
    dc_l_le FILEX_START,(FILEX_END-FILEX_START)  
    cnop    0,2048



FILEZ_START
    incbin FileZ.BIN
FILEZ_END
    cnop    0,2048

FILEX_START
FILEY_START
    incbin FileY.BIN
FILEY_END
    cnop    0,2048
    incbin FileX.bin
FILEX_END

Note: This otherwise good looking assembler cannot handle whitespace in expressions. I'd love to write (.value >> 24) & $FF but this led to irritating error messages. It took some time to find the reason.

EDIT:

The command line for this suggestion is:

vasmm68k_mot.exe -Fbin -o test.bin -L test.lst test.S

The specific output format is not important, I think, and the listing is generated just to see that the solution works. The "S" source extension is deliberately chosen out of habbits.

The assembler shows these versions, I used the provided binary on Win10:

vasm 1.8k (c) in 2002-2021 Volker Barthelmann
vasm M68k/CPU32/ColdFire cpu backend 2.4 (c) 2002-2021 Frank Wille
vasm motorola syntax module 3.15a (c) 2002-2021 Frank Wille
vasm binary output module 2.1 (c) 2002-2021 Volker Barthelmann and Frank Wille

To avoid relocation errors, an org pseudo instruction is added just for demonstration, but the real thing is most probably allocated in other ways.

Answer 3

Unless you know something about the file data, I think the simple answer is 'no'. If the data was bytes, they would be in the same order. If it was 16-bit words, they would be swapped in pairs. If it was 32-bit words, then 4 bytes would be reversed.

I would imagine you could look at a dump of the data and make a reasonable guess most of the time. But there is no simple swapEndian.exe tool that can do it for you.

[Off-topic, but I am now intrigued: I wonder how good a neural net might be at guessing and swapping?]