5

When using MASM 6.15 under DOSBox whenever I try to do:

mov al, [cx]

or

mov al, [cx + bx]

or

mov al, [cx + 4]

I am getting the error: 'Only Base or Index Registers are allowed'

But then again, let's say I have an array var1. If I do:

.model small
.stack 4096

.data
 var1 BYTE 1, 2, 3, 4, 5, 6, 7, 8, 9

.code
main proc
mov ax, @data
mov ds, ax

mov cx, 5
mov al, [var1 + cx]

mov ah, 4Ch
int 21h
main endp
end main

It works perfectly fine. Why doesn't it give the same error as above? CX is not a base or index register.

What is the whole working essence of the [] operator?

Michael Petch
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Parker Queen
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  • Possible duplicate of [16-bit Assembly: cannot deref some registers](https://stackoverflow.com/questions/52740507/16-bit-assembly-cannot-deref-some-registers) – fuz Oct 22 '18 at 17:39
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    `mov al, [var1 + cx]` should not actually work. `cx` is not allowed as a base or index register (base registers are `bx` and `bp`, index registers are `si` and `di`. Either or both can be left out). I suspect you misinterpreted the assembler's output here. – fuz Oct 22 '18 at 17:41
  • @fuz Check the code I pasted above. It assembles alright without any error. I am using masm615 (DOSBox). It is not the duplicate, I am not saying what registers should work. I am asking a code specific question as to why is it working when it shouldn't. – Parker Queen Oct 22 '18 at 17:51
  • What code does the assembler generate for that? – prl Oct 22 '18 at 17:56
  • @prl What do you mean? Do you mean the binary? How can I provide it? – Parker Queen Oct 22 '18 at 17:57
  • @ParkerQueen Thank you for the clarification. Close vote retracted. You might want to update the title of the question to make this clearer. Something like “Why does mov al, [var1 + cx] assemble despite cx not being a valid base or index register?” – fuz Oct 22 '18 at 18:03
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    @ParkerQueen Use a disassembler of choice to disassemble the binary. In DOS `debug` you can use the `u` command to disassemble code. – fuz Oct 22 '18 at 18:05
  • Look at the assembler listing file. – prl Oct 22 '18 at 18:07

1 Answers1

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This is a bug in some versions of MASM. There are cases like these where MASM will try to incorrectly encode an instruction that shouldn't be possible. It should have thrown an error trying to encode this, as CX can't be used as a base or index in 16-bit addressing.

mov al, [var1 + cx]

Instead of generating an error it incorrectly generates it as:

mov bh, var1[bx+si]

The invalid instruction is encoded as:

8A B8 xx xx

Where xx xx is the offset of var1.

Michael Petch
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  • Can you recommend a version of masm which does not do this? I am using masm615 right now. – Parker Queen Oct 22 '18 at 18:27
  • @ParkerQueen If you are trying to create 16-bit executables to be used by DOS then MASM615 is about the latest version where that is possible. If you switched to another assembler it could solve that. JWASM in general is close to MASM and will throw an error in this case. JWASM can be found here: https://sourceforge.net/projects/jwasm/files/ . You can assembly an assembly file with `jwasm -mz file.asm` that will generate `file.obj` that can be then linked with a DOS segmented linker. – Michael Petch Oct 22 '18 at 18:35
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    @MichaelPetch: I've build OP's example with ML version 10.00.40219.01 (with /OMF switch) - and the bug still exists. MASM 5.1 and older issue the correct error message. TASM also shows the correct behavior. – rkhb Oct 22 '18 at 19:02
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    Even MASM 14.10.25019.0 has this bug, so it looks like it exists in MASM 6 or later and probably will never be fixed. (Otherwise you can use the latest version of MASM to build 16-bit MS-DOS executables, the trick is finding a linker that can.) – Ross Ridge Oct 22 '18 at 19:32
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    @RossRidge : Likely you have to go back to 5.1 as rkhb suggests but then that won't run under 64-bit Windows since those versions were for DOS and OS/2. My recommendation of JWASM is that it was based on Watcom's MASM compatible assembler; assembler runs on Win32+ platforms; doesn't exhibit this bug. As for a linker that can generate 16-bit executables, that can be found here: http://www.capp-sysware.com/downloads/link16.exe – Michael Petch Oct 22 '18 at 19:39