A brief discussion on ARM pseudo-instruction address reading

1. ADR pseudo-instruction --- small range address reading

The ADR pseudo-instruction reads the address value based on the PC relative offset or the address value based on the register relative offset into the register. When the assembler compiler compiles the source program, the ADR pseudo-instruction is replaced by a suitable instruction by the compiler. Usually, the compiler uses an ADD instruction or a SUB instruction to implement the function of the ADR pseudo-instruction. If it cannot be implemented with one instruction, an error will be generated and the compilation will fail.

ADR pseudo-instruction format: ADR{cond} register, exp

The value range of the address expression expr is:

When the address value is byte aligned, its fetch range is: +255 ~ 255B;

When the address value is word-aligned, the range of its fetch is: -1020 ~ 1020B;

2. ADRL pseudo-instruction ---- medium range address reading

The ADRL pseudo-instruction reads the address value based on the PC relative offset or the address value based on the register relative offset into the register. It can read a wider range of addresses than the ADR pseudo-instruction. When the assembler compiler compiles the source program, the ADRL pseudo-instruction is replaced by two appropriate instructions by the compiler. If it cannot be implemented with two instructions, an error will occur and the compilation will fail.

ADRL pseudo-instruction format: ADRL{cond} register, expr

The value range of the address expression expr is:

When the address value is byte aligned, the range of its fetch is: -64K～64K;

When the address value is word-aligned, the range of its fetch is: -256K～256K;

3. LDR pseudo-instruction-----large range of address reading

The LDR pseudo-instruction is used to load a 32-bit immediate value or an address value into a specified register. When assembling a source program, the LDR pseudo-instruction is replaced by a suitable instruction by the compiler. If the loaded constant does not exceed the range of MOV or MVN, the MOV or MVN instruction is used to replace the LDR pseudo-instruction. Otherwise, the assembler puts the constant into a literal pool and uses a program-relative offset LDR instruction to read the constant from the literal pool.