1. ARM Assembly Overview

1. Assembly usage location

Compilation location:

-- Startup code: Bootloader initializes the CPU and coprocessor, etc., and the C language operating environment is not established at this time. At this time, assembly language is used to perform initialization operations;

-- Efficiency requirements: high assembly efficiency. In the Linux kernel, assembly is required where there are special requirements for efficiency.

2. Compilation classification

(1) ARM standard assembly

ARM Standard Assembly Introduction:

-- Usage scenario: Applicable to ARM's assembler, suitable for use on Windows platform, such as ADS;

(2) GNU Assembler

GNU Assembler Introduction:

-- Usage scenario: Assembler for cross-compilation toolchain on Linux platform;

3. ARM assembler framework

ARM Assembly Framework:

-- ARM assembly framework example:

.section .data
	
.section .bss
	
.section .text
.global _start
_start:
	

-- Program entry: "_start:" is the entry of the assembler program, equivalent to main();

-- Mark the entry: Use ".global _start" to mark the program entry so that the outside world can recognize that this is the program entry;

-- Mark the code section: ".section .text" marks this as a code section;

-- Mark the bss segment: Use ".section .bss" to mark the bss segment. If there is no bss segment and data segment, start directly from .text;

4. Build an assembly development and debugging environment

(1) Assembler preparation

Program code:

-- Define code section: .text;

-- Define program entry: .globl _start;

-- Code example:

.text
.globl _start
_start:
	mov r1,#1
	mov r2,#2
	mov r3,#3

Makefile code:

-- Link elf format file: Set the program start position 6410 board is 0x50008000 address;

-- Specify the program start address in arm-linux-ld: -Ttext 50008000;

-- If you use a linker script to specify the address: Note that the third line specifies the program start address;

SECTIONS
{
	. = 0x50008000;

	. = ALIGN(4);
	.text :
	{
		led.o (.text)
		*(.text)
	}

	. = ALIGN(4);
	.rodata : { *(SORT_BY_ALIGNMENT(SORT_BY_NAME(.rodata*))) }

	. = ALIGN(4);
	.data : { *(.data) }


	. = ALIGN(4);
	.bss (NOLOAD) : { *(.bss) . = ALIGN(4); }
}

-- Code example:

all:start.o
	arm-linux-ld -Ttext 0x50008000 -o start.elf $^

%.o:%.S
	arm-linux-gcc -g -o $@ $^ -c

clean:
	rm -rf *.o *.elf

(2) Start JLink debugging

JLink debug start:

-- Ensure the driver is installed: Note that you need to install the Windows driver;

-- Connect JLink: Connect JLink in the lower right corner of the virtual machine;

-- Start JLinkGDBServer:

[root@localhost JLink_Linux_V434a]# ./JLinkGDBServer 
SEGGER J-Link GDB Server V4.34a

JLinkARM.dll V4.34a (DLL compiled Aug 31 2011 11:51:40)

Listening on TCP/IP port 2331

J-Link connected
Firmware: J-Link ARM V8 compiled Aug 24 2011 17:23:32
Hardware: V8.00
S/N: 17935099
Feature(s): RDI,FlashDL,FlashBP,JFlash

J-Link found 2 JTAG devices, Total IRLen = 5
JTAG ID: 0x07B76F0F (ARM11)

(2) Eclipse debugging environment

Build eclipse debugging environment:

-- Import project: Select Makefile Project With Existing Code;

-- Select the imported code location:

-- clean code: select "Project" --> "Clean";

-- Build project: Select "Menu" --> Project --> Build All option;

--Configure Debug parameters:

-- Execute debugging: Press F6 to step through the debugging. You can view the register values ​​in the Register view. You can see that r1 and r2 are assigned 1 and 2.

2. ARM instruction classification

ARM Assembly Manual:

The difference between GNU assembly and ARM standard assembly: The manual above is the ARM standard assembly manual, and we wrote the GNU assembly manual, which is somewhat different;

-- Case difference: ARM standard assembly is all uppercase, GNU assembly can be lowercase;

1. Arithmetic and logical instructions

(1) MOV instruction

Introduction to MOV instruction: assignment operation;

-- Syntax format: MOV , ;

-- Syntax analysis: dest is the destination register, op1 can be an immediate value, a register, an address, etc., which is equivalent to dest = op1;

Assembler comments: Use the "@" symbol to add comments in the assembly;

Sample code:

.text
.global _start
_start:

@mov command example
mov r1, #8 @Assign 8 to r1
mov r2, r1 @assign the value in r1 to r2
mov r3, #10 @Assign 10 to register r3

(2) MVN command

MVN instruction introduction: negate assignment operation;

-- Syntax format: MVN , ;

-- Syntax analysis: Invert the operand op1 and assign it to dest;

Command example:

-- Code:

.text
.global _start
_start:

@mvn command example
mvn r1, #0b10 @0b10 Binary number inversion, assign to r1
mvn r2, #5 @5 decimal number inversion, assign to r2
mvn r3, r1 @Assign the value of register r1 to r3

(3) SUB instruction

SUB instruction introduction: subtraction operation;

-- Syntax format: SUB , , ;

-- Syntax analysis: dest stores the subtraction result, op1 is the subtrahend, op2 is the minuend, dest = op1 - op2;

-- Note: dest op1 cannot use immediate data, but op2 can use immediate data;

Code example:

.text
.global _start
_start:

@sub directive examples
@sub r1, #4, #2 Incorrect example, the subtraction number cannot be an immediate number, it must be a register
mov r2, #4
sub r1, r2, #4
mov r0, #1
sub r3, r1, r0

(4) ADD instruction

ADD instruction introduction: addition operation;

-- Syntax format: ADD , , ;

-- Syntax analysis: dest stores the result of addition, op1 and op2 are the two numbers to be added, dest = op1 + op2;

-- Note: dest op1 cannot use immediate data, but op2 can use immediate data;

Code example:

@add directive example
mov r2, #1
add r1, r2, #3

(5) AND instruction

AND instruction introduction: logical and operation;

-- Syntax format: AND , , ;

-- Syntax analysis: dest stores the result of logical AND operation, op1 and op2 are two numbers to be ANDed, dest = op1 & op2;

-- Note: dest op1 cannot use immediate data, must use registers, op2 can use immediate data;

Code example:

.text
.global _start
_start:

@and Directive Examples
mov r1, #5
and r2, r1, #0

mov r1, #5
mov r2, r1, #1

(6) BIC instruction

BIC instruction introduction: bit clear instruction operation;

-- Syntax format: AND , , ;

-- Syntax analysis: dest stores the result of clearing the bit, op1 is the object to be cleared, and op2 is the mask;

-- Example: "bic r0, r0, #0b1011", clear bits 0, 1, and 3 in r0, keep the rest of the bits unchanged, and put the result in r0;

-- Note: dest op1 cannot use immediate data, must use registers, op2 can use immediate data;

-- Binary representation: % in the mask represents binary in standard assembly, but cannot be used in GNU assembly. GNU assembly uses 0b to represent binary;

Code example:

.text
.global _start
_start:

@bic directive examples
mov r1, #0b101011
bic r2, r1, #0b101 @ Clear bits 0 and 2 of r1

2. Comparison Instructions

(1) CMP instruction

CMP instruction introduction: comparison instruction;

-- Syntax format: CMP , ;

-- Syntax analysis: There are three comparison results: op1 > op2 (CPSR N = 0), op1 = op2 (CPSR Z = 1), op1 < op2 (CPSR N = 1), the result is placed in the CPSR register;

Code example:

.text
.global _start
_start:

@cmp directive example
mov r1, #2
cmp r1, #1

mov r1, #2
cmp r1, #3

mov r1, #2
cmp r1, #2

(2) TST instruction

TST instruction introduction: comparison instruction;

-- Syntax format: TST , ;

-- Syntax analysis: op1 and op2 are bitwise ANDed, the result affects the CPSR register, if the result is not 0, CPSR's Z = 0, if the result is 0, Z = 1;

Code example:

.text
.global _start
_start:

@cmp directive example
mov r1, #0b101
tst r1, #0b001 @The bitwise AND result is 0b1, the result is not 0, CPSR Z = 0

mov r1, #0b101
tst r1, #0b10 @The bitwise AND result is 0, the result is not

3. Branch Instructions

(1) B instruction

B Instruction Introduction: Branch instruction;

-- Syntax format: B{condition} address;

-- Syntax analysis: If the condition is met, jump to the address location, if the condition is not met, execute the following statement, if there is no condition, it is 100% executed;;

Code example:

-- Conditional analysis: gt is a greater than condition, if r1 > r2, go to the conditional branch, otherwise continue to execute the next one;

.text

.global _start

_start:

@b branch instruction example

mov r1, #6

mov r2, #5

cmp r1, r2 @Compare the values ​​in r1 and r2

@b can be followed by a condition. {condition} in {} can be added or not. If there is no condition, it will be executed unconditionally 100%.

@gt is a greater than condition instruction. If the condition is met, it will jump to branch1. If not, it will execute the following instruction.

bgt branch1

add r3, r1, r2

b end @In order not to execute branch1 operation here, jump directly to end execution

branch1:

sub r3, r1, r2

end:

nop

(2) BL instruction

BL instruction introduction: branch instruction with connection;

-- Syntax format: BL{condition} address;

-- Syntax analysis: If the condition is met, jump to the address location, if the condition is not met, execute the following statement, if there is no condition, it is 100% executed;;

Code example:

.text

.global _start

_start:

@bl Branch instruction example with connection