Expression Classes

The built-in assembler divides expressions into three classes: registers, memory references, and immediate values.

An expression that consists solely of a register name is a register expression. Examples of register expressions are AX, CL, DI, and ES. Used as operands, register expressions direct the assembler to generate instructions that operate on the CPU registers.

Expressions that denote memory locations are memory references. Delphi's labels, variables, typed constants, procedures, and functions belong to this category.

Expressions that aren't registers and aren't associated with memory locations are immediate values. This group includes Delphi's untyped constants and type identifiers.

Immediate values and memory references cause different code to be generated when used as operands. For example, const

Count: Integer;

asm MOV MOV MOV MOV end;

EAX,Start EBX,Count ECX,[Start] EDX,OFFSET Count

  • MOV EAX,xxxx }
  • MOV EBX,[xxxx] }
  • MOV ECX,[xxxx] } { MOV EDX,xxxx }

Because Start is an immediate value, the first MOV is assembled into a move immediate instruction. The second MOV, however, is translated into a move memory instruction, as Count is a memory reference. In the third MOV, the brackets convert Start into a memory reference (in this case, the word at offset 10 in the data segment). In the fourth MOV, the OFFSET operator converts Count into an immediate value (the offset of Count in the data segment).

The brackets and OFFSET operator complement each other. The following asm statement produces identical machine code to the first two lines of the previous asm statement.

MOV EAX,OFFSET [Start]

MOV EBX,[OFFSET Count]

end;

Memory references and immediate values are further classified as either relocatable or absolute. Relocation is the process by which the linker assigns absolute addresses to symbols. A relocatable expression denotes a value that requires relocation at link time, while an absolute expression denotes a value that requires no such relocation. Typically, expressions that refer to labels, variables, procedures, or functions are relocatable, since the final address of these symbols is unknown at compile time. Expressions that operate solely on constants are absolute.

The built-in assembler allows you to carry out any operation on an absolute value, but it restricts operations on relocatable values to addition and subtraction of constants.

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