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Table of Contents
2. TMT Pascal Language Description
2.3. Built-in Assembler
2.3.2. Assembler Procedure
2.3.2. Assembler Procedure
The built-in assembler can also be used to write entire procedures
in assembler language. Such procedures should have the assembler
keyword appended after a procedure header.
function MultBy9(X: Longint):Longint; assembler;
asm
MOV EAX,[X]
LEA EAX,[EAX*8+EAX]
end;
The function above used i80386 index scaling feature to implement
very fast multiplication by 9.
The assembler procedures differ from the standard Pascal procedures
in the following ways:
No Return variable
There is no return variable. You must return the function results
in an appropriate register. More precisely,
- Ordinal values are returned in AL (8-bit values),
AX (16-bit values), or EAX (32-bit values).
- Real values are returned in DX:BX:AX.
- Floating point (8087) values are returned in ST(0).
- Pointers are returned in EAX.
- Strings are returned in a temporary location pointed
by the @Result symbol.
Structured variables
Structured arguments (i.e. strings, objects, records) are not copied
into the local variables. They should be treated as var parameters.
Stack Frame
Assembler procedures have no stack frames if they have no arguments and no
local symbols. Generally, the stack frame supplied by the built-in
assembler is PUSH EBP // Appears if locals + params >0
MOV EBP,ESP // Appears if locals + params >0
SUB ESP, locals // Appears if locals + params >0
...
LEAVE // Appears if locals + params >0
RETN params // Always appears
Here Locals is the total size of local parameters,
Params is the total size of procedure parameters.
Register Preservations
Assembler code should preserve the following registers:
DS, CS, SS, ES, EBP, and ESP. All other registers can be destroyed.
Notice the inclusion of the ES register.
TMT Pascal always assumes that ES is equal to DS.
Furthermore, you should not change segment, page, and interrupt tables,
as well as the control, debug and test registers, unless you are thoroughly
familiar with 386 protected mode architecture. The privileged instructions
like LGDT and LIDT are supported by built-in assembler. However, avoid using
them unless you know exactly what you are doing.
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