Convert an as86 to fasm

Question

I'm trying to convert this bit of assembly for as86 to fasm. I have 3 questions:

1) Why seg es given an error: illegal instruction. this is not valid in 16-bit?

2) Is mov byte [0],0x41(FASM syntax) exactly equivalent to mov [0],#0x41(as86 syntax)? if isn't,can you show me the equivalent to?

3) Why entry start give an error in FASM?

Here's the assemblies codes:

as86

entry start
start:
       mov ax,#0xb800
       mov es,ax
       seg es
       mov [0],#0x41
       seg es
       mov [1],#0x1f
 loop1: jmp loop1

and the fasm version that I wrote:

FASM

use16
format binary

start:
    mov ax,0xb800
    mov es,ax
    seg es
    mov byte [0],0x41
    seg es
    mov byte [1],0x1f
loop1:  jmp loop1

GJ. · Accepted Answer · 2013-01-20T09:28:41.467

1

Correct syntax is:

mov byte [es:0],0x41    ;I'm not sure if this instruction is supported under 16 bit CPU

or

push bx 
mov  bx,0   ;you can use also: xor  bx, bx
mov  byte [es:bx],0x41
pop  bx

edited Jan 20 '13 at 09:28

answered Jan 19 '13 at 22:52

GJ.

Is this assembly fasm's syntax? doesn't works too: `error: reserved word used as symbol.` – Jack Jan 19 '13 at 22:57
GJ: Yes, `mov byte [es:0],0x41` is a valid 16-bit x86 instruction, NASM encodes it as `26 C6 06 00 00 41`. – nrz Jan 19 '13 at 22:57
I know that it's valid 16-bit x86 instruction. I'm asking for if is FASM sytax. – Jack Jan 19 '13 at 22:58
@Jack I meant my comment as a response to GJ to "I'm not sure if this instruction is supported under 16 bit CPU"... – nrz Jan 19 '13 at 23:00
@nrz: My apologies. I had not seen the 'GJ:' part in the start of string. – Jack Jan 19 '13 at 23:02
@GJ.: But isn't working for me. I think that isn't valid under 16-bit CPU? – Jack Jan 19 '13 at 23:04
@Jack: you can try with second example. – GJ. Jan 19 '13 at 23:06
@GJ.: I had tried the second only. `byte [es:ax]` fails but `byte [es:0]` is fine. Thanks. – Jack Jan 19 '13 at 23:08
@GJ.: So by using `mov byte [es:0],0x41` don't need a `seg es`? – Jack Jan 19 '13 at 23:10
@Jack: No. The es prefix must be after bracket... [es:0] – GJ. Jan 19 '13 at 23:13
`ax` cannot be used as a memory address of an instruction operand. Only `bx`, `bp`, `si`, `di` can. – Alexey Frunze Jan 20 '13 at 02:21
@GJ.: worked. Is `mov byte [es:1],0x1f` equvalent to `*(es + 1) = 0x1f;` in C? – Jack Jan 21 '13 at 01:04
@Jack: Check with disassembler! – GJ. Jan 21 '13 at 07:26

score 0 · Answer 2 · answered Jan 19 '13 at 21:43

0

seg es looks fishy. Try following:

mov byte ptr es:[0],0x41

answered Jan 19 '13 at 21:43

Seva Alekseyev

doesn't works for me: `s.asm [21]: mov byte ptr es:[0],0x41 error: invalid expression.` – Jack Jan 19 '13 at 22:02

2 Answers2