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Arm Compiler armasm User Guide : THUMB directive

THUMB directive

The THUMB directive instructs the assembler to interpret subsequent instructions as T32 instructions, using the UAL syntax.

Note

Not supported for AArch64 state.

Syntax

THUMB

Usage

In files that contain code using different instruction sets, the THUMB directive must precede T32 code written in UAL syntax.

If necessary, this directive also inserts one byte of padding to align to the next halfword boundary.

This directive does not assemble to any instructions. It also does not change the state. It only instructs armasm to assemble T32 instructions as appropriate, and inserts padding if necessary.

Example

This example shows how you can use ARM and THUMB directives to switch state and assemble both A32 and T32 instructions in a single area.

        AREA ToT32, CODE, READONLY      ; Name this block of code
        ENTRY                           ; Mark first instruction to execute
        ARM                             ; Subsequent instructions are A32 
start
        ADR     r0, into_t32 + 1        ; Processor starts in A32 state 
        BX      r0                      ; Inline switch to T32 state
        THUMB                           ; Subsequent instructions are T32
into_t32
        MOVS    r0, #10                 ; New-style T32 instructions
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