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4.2 Accessing C global variables from assembly code

Global variables can only be accessed indirectly, through their address. To access a global variable, use the IMPORT

directive to import the global and then load the address into a register. You can access the variable with load and

store instructions, depending on its type.

For unsigned variables use:

• LDRB/STRB for char

• LDRH/STRH for short (Use two LDRB/STRB instructions for Architecture 3)

• LDR/STR for int.

For signed variables, use the equivalent signed instruction, such as LDRSB and LDRSH.

Small structures of less than eight words can be accessed as a whole using the LDM and STM instructions. Individual

members of structures can be accessed by a load or store instruction of the appropriate type. You must know the

offset of a member from the start of the structure in order to access it.

Example 4-6 loads the address of the integer global globvar into r1, loads the value contained in that address into

r0, adds 2 to it, then stores the new value back into globvar.

Example 4-6 Address of global

AREA globals,CODE,READONLY

EXPORT asmsubroutine

IMPORT globvar

asmsubroutine

LDR r1, =globvar ; read address of globvar into

; r1 from literal pool

LDR r0, [r1]

ADD r0, r0, #2

STR r0, [r1]

MOV pc, lr

END

Mixing C, C++, and Assembly Language

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ARM VERSION 1.2 Guía de usuario Pagina 39