Free Pascal supports Intel syntax for the Intel family of Ix86 processors in its asm blocks.
The Intel syntax in your asm block is converted to AT&T syntax by the compiler, after which it is
inserted in the compiled source. The supported assembler constructs are a subset of the normal
assembly syntax. In what follows we specify what constructs are not supported in Free Pascal, but
which exist in Turbo Pascal:
- The TBYTE qualifier is not supported.
- The & identifier override is not supported.
- The HIGH operator is not supported.
- The LOW operator is not supported.
- The OFFSET and SEG operators are not supported. Use LEA and the various Lxx
- Expressions with constant strings are not allowed.
- Access to record fields via parenthesis is not allowed
- Typecasts with normal pascal types are not allowed, only recognized assembler typecasts are
mov al, byte ptr MyWord -- allowed,
mov al, byte(MyWord) -- allowed,
mov al, shortint(MyWord) -- not allowed.
- Pascal type typecasts on constants are not allowed. Example:
const s= 10; const t = 32767;
in Turbo Pascal:
mov al, byte(s) -- useless typecast.
mov al, byte(t) -- syntax error!
In Free Pascal the compiler emits a syntax error in both cases will give out a syntax
- Expressions with constants only are not allowed (in all cases they do not work in protected
mode, e.g. under linux i386). Examples:
mov al,byte ptr ['c'] -- not allowed.
mov al,byte ptr [100h] -- not allowed.
(This is due to the limitation of the GNU Assembler).
- Brackets within brackets are not allowed
- Expressions with segment overrides fully in brackets are currently not supported, but they
can easily be implemented in the compiler if requested. Example:
mov al,[ds:bx] -- not allowed
- Possible allowed indexing are as follows:
Where Sreg is optional and specifies the segment override. Notes:
- The order of terms is important contrary to Turbo Pascal.
- The Scaling value must be a value, and not an identifier to a symbol. Examples:
const myscale = 1;
mov al,byte ptr [esi+ebx*myscale] -- not allowed.
mov al, byte ptr [esi+ebx*1]
- Possible variable identifier syntax is as follows: (Id = Variable or typed constant
Possible fields are as follows:
- ID.subfield.subfield …
- [ref].ID.subfield.subfield …
- [ref].typename.subfield …
- Local labels: Contrary to Turbo Pascal, local labels, must at least contain one character after
the local symbol indicator. Example:
@: -- not allowed
@1: -- allowed
- Contrary to Turbo Pascal, local references cannot be used as references, only as
lds si,@mylabel -- not allowed
- Contrary to Turbo Pascal, SEGCS, SEGDS, SEGES and SEGSS segment overrides are presently
not supported. (This is a planned addition though).
- Contrary to Turbo Pascal where memory sizes specifiers can be practically anywhere, the
Free Pascal Intel inline assembler requires memory size specifiers to be outside the brackets.
mov al,[byte ptr myvar] -- not allowed.
mov al,byte ptr [myvar] -- allowed.
- Base and Index registers must be 32-bit registers. (limitation of the GNU Assembler).
- XLAT is equivalent to XLATB.
- Only Single and Double FPU opcodes are supported.
- Floating point opcodes are currently not supported (except those which involve only floating
The Intel inline assembler supports the following macros:
- represents the function result return value.
- represents the object method pointer in methods.