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Assembler User Guide
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Overview of the Assembler Overview of the ARM Architecture Structure of Assembly Language Modules Writing ARM Assembly Language Condition Codes Using the Assembler 
Symbols, Literals, Expressions, and Operators Symbol naming rules Variables Numeric constants Assembly time substitution of variables Register-relative and PC-relative expressions Labels Labels for PC-relative addresses Labels for register-relative addresses Labels for absolute addresses Local labels Syntax of local labels String expressions String literals Numeric expressions Numeric literals Floating-point literals Logical expressions Logical literals Unary operators Binary operators Multiplicative operators String manipulation operators Shift operators Addition, subtraction, and logical operators Relational operators Boolean operators Operator precedence Difference between operator precedence in armasm a VFP ProgrammingRegister-relative and PC-relative expressions
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| Home > Symbols, Literals, Expressions, and Operators > Register-relative and PC-relative expressions | |||
Addresses can be represented as a register-relative or PC-relative expression.
A register-relative expression evaluates to a named register combined with a numeric expression.
A PC-relative expression is written in source code as the PC or a label combined with a numeric expression. It can also be expressed in the form [PC, #number]. It is represented in the instruction as the PC value plus or minus a numeric offset. The assembler calculates the required offset from the label and the address of the current instruction. If the offset is too big, the assembler produces an error.
It is recommended to write PC-relative expressions using labels rather than PC because the value of PC depends on the instruction set.
Note
In ARM state, the value of the PC is the address of the current instruction plus 8 bytes.
In Thumb state:
For
B,BL,CBNZ, andCBZinstructions, the value of the PC is the address of the current instruction plus 4 bytes.For all other instructions that use labels, the value of the PC is the address of the current instruction plus 4 bytes, with bit[1] of the result cleared to 0 to make it word-aligned.
Example
LDR r4,=data+4*n ; n is an assembly-time variable
; code
MOV pc,lr
data DCD value_0
; n-1 DCD directives
DCD value_n ; data+4*n points here
; more DCD directives
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