Chipset directives ...

[James Ladd]

At the top of my code I can put

Code Select Expand

.386

What are the supported values and do they have any effect on the code generated ?

[thomasantony]

Hi,
   I think this specifies the instruction set you use. With .386 you can use 386 instructions and 32-bit registers etc. But you cannot use some instructions like BSWAP which is available only in 486.

Thomas :U

[MichaelW]

MSDN: Microsoft Macro Assembler Reference, Directives Reference

[Mirno]

The directive changes available instructions, so MASM will throw an error if you use later instructions.
You can specify several processor directives in the same bit of code, so you may find that you get code that does the initial processor detection under .386, then moves to .586 & .MMX when it's detected an appropriate processor. It's for safety really.

It also changes the instruction timings if you generate a list file "ml /Fl<name.txt> <asmfile.asm>". Although note that .686 does not generate instruction timings because it's out of order execution does not lend itself to easy clock cycle numbers.

AFAIK the directive won't change instructions generated... Although I could be wrong (work arounds may be in place on a specific generation for instance).

Mirno

[AeroASM]

AFAIK the directive won't change instructions generated...

With .286 it puts a 66 prefix on 32-bit registers (eax)

With .686 it puts a 66 prefix on 16-bit registers (ax)

[MichaelW]

AFAIK the directive won't change instructions generated...

With .286 it puts a 66 prefix on 32-bit registers (eax)

With .686 it puts a 66 prefix on 16-bit registers (ax)

Although this would not be obvious looking at a disassembled listing, there is no direct connection between the processor directive and the default operand size, and for any processor selection below .386 MASM will refuse to assemble 32-bit instructions. The operand size prefix (66h) overrides the default operand size. For 32-bit protected mode the default operand size is 32 bits (determined by the D bit in the executable segment descriptors). For real or V86 mode the default operand size is 16 bits.