fs add word[$],$ != add word[fs:$],$

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Index > Compiler Internals > fs add word[$],$ != add word[fs:$],$

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revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20583
Location: In your JS exploiting you and your system

revolution 18 Mar 2023, 01:18

Code:

fs add word[$],$ ; <-- what is the value of $ here?

The expected output is:

Code:

64 83 06 00 00 00

But the fs prefix is processed then ignored and the actual output is:

Code:

64 83 06 01 00 01

The same goes for any prefixes, lock included.

This can be partly overcome by moving fs inside the brackets

Code:

add word[fs:$],$

But that trick can't be used for lock.

Code:

lock add byte[$],$ ; <-- not the address expected

18 Mar 2023, 01:18

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8376
Location: Kraków, Poland

Tomasz Grysztar 18 Mar 2023, 07:32

Yes, explicit prefixes were implemented in fasm in a very simplistic way. And apparently I did not consider it an issue, because when implementing fasm-compatible x86 encoders for fasmg, I did reproduce this exact behavior. Well, at least they are consistent.

18 Mar 2023, 07:32

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