User guide

ManualsBrandsARM ManualsComputer HardwareVERSION 1.2

Note

Using physical register names is not recommended, because it constrains compiler register allocation and can cause

less efficient code to be generated. It is usually better to declare C local variables and use these as operands in

inline assembler.

Constants

The constant expression specifier # is optional. If it is used, the expression following it must be constant.

Instruction expansion

The constant in instructions with a constant operand is not limited to the values allowed by the instruction. Instead,

such an instruction is translated into a sequence of instructions with the same effect. For example:

ADD r0, r0, #1023

might be translated into:

ADD r0, r0, #1024

SUB r0, r0, #1

With the exception of coprocessor instructions, all ARM and Thumb instructions with a constant operand support

instruction expansion. In addition, the MUL instruction can be expanded into a sequence of adds and shifts when the

third operand is a constant.

The effect of updating the CPSR by an expanded instruction is:

• arithmetic instructions set the NZCV flags correctly.

• logical instructions:

— set the NZ flags correctly

— do not change the V flag

— corrupt the C flag.

Labels

C and C++ labels can be used in inline assembler statements. C and C++ labels can be branched to by branch

instructions only in the form:

B{cond} label

You cannot branch to C or C++ labels using BL.

Storage declarations

All storage can be declared in C or C++ and passed to the inline assembler using variables. Therefore, the storage

declarations that are supported by armasm are not implemented.

SWI and BL instructions

SWI and BL instructions must specify exactly the calling standard used. Three optional register lists follow the normal

instruction fields. The register lists specify:

• the registers that are the input parameters

• the registers that are output parameters after return

• the registers that are corrupted by the called function.

For example:

SWI{cond} swi_num, {input_regs}, {output_regs}, {corrupted_regs}

BL{cond} function, {input_regs}, {output_regs}, {corrupted_regs}

An omitted list is assumed to be empty, except that BL always corrupts ip, and lr. The default corrupted list for BL is

r0-r3.

The register lists have the same syntax as LDM and STM register lists. If the NZCV flags are modified you must

specify PSR in the corrupted register list.

4.1.3 Differences between the inline assemblers and armasm

There are a number of differences and restrictions between the assembly language accepted by the inline

assemblers and the assembly language accepted by the ARM assembler. For the inline assemblers:

• You cannot get the address of the current instruction using dot notation (.) or {PC}.

Mixing C, C++, and Assembly Language