Program structure in ARM assembly language

In ARM (Thumb) assembly language programs, code is organized in program segments. A segment is a relatively independent sequence of instructions or data with a specific name. Segments can be divided into code segments and data segments. The content of the code segment is the execution code, and the data segment stores the data needed when the code is running. An assembly program should have at least one code segment. When the program is long, it can be divided into multiple code segments and data segments. Multiple segments will eventually form an executable image file when the program is compiled and linked.

An executable image file usually consists of the following parts:

— One or more code segments, the properties of the code segments are read-only.

— Zero or more data segments containing initialized data. The data segments are readable and writable.

— Zero or more data segments that do not contain initialized data, and the attributes of the data segments are readable and writable.

The linker arranges each segment in the corresponding position in the memory according to the system default or user-defined rules. Therefore, the relative positions of the segments in the source program and the relative positions of the segments in the executable image file are generally different.

The following is the basic structure of an assembly language source program:

AREA Init ， CODE ， READONLY

ENTRY

Start

LDR R0 ， =0x3FF5000

LDR R1 ， 0xFF

STR R1 ， [R0]

LDR R0 ， =0x3FF5008

LDR R1 ， 0x01

STR R1 ， [R0]

┉┉

END

In assembly language programs, the AREA directive is used to define a segment and describe the relevant attributes of the defined segment. This example defines a code segment named Init with read-only attributes. The ENTRY directive identifies the entry point of the program, followed by a sequence of instructions. The end of the program is the END directive, which tells the compiler the end of the source file. Each assembly program segment must have an END directive to indicate the end of the code segment. Subroutine calls in ARM assembly language

 
 
  
 

In ARM assembly language programs, subroutine calls are generally implemented through BL instructions. In the program, use the instruction:

BL Subroutine name

The subroutine call is completed.

This instruction completes the following operations when executed: the return address of the subroutine is stored in the link register LR, and the program counter PC points to the entry point of the subroutine. When the subroutine is executed and needs to return to the calling place, it only needs to copy the return address stored in LR to the program counter PC. When calling a subroutine, it can also complete the transfer of parameters and the return of calculation results from the subroutine, which can usually be completed using registers R0 to R3.

The following is the basic structure of an assembly language source program that uses the BL instruction to call a subroutine:

AREA Init ， CODE ， READONLY

ENTRY

Start

LDR R0 ， =0x3FF5000

LDR R1 ， 0xFF

STR R1 ， [R0]

LDR R0 ， =0x3FF5008

LDR R1 ， 0x01

STR R1 ， [R0]

BL PRINT_TEXT

┉┉

PRINT_TEXT

┉┉

MOV PC，BL

┉┉

END