ARM Learning Notes 2 - Branch Jump Instructions

1. Arm instruction condition codes and condition mnemonics

2. Jump instruction B

　　1. Function

　　　　Jump instruction B causes the program to jump to the specified address to execute the program (the jump range is PC-32MB to PC+32MB)

　　2. Instruction format (Note: If there is a condition after B, the condition is immediately after B, without a space)

　　　　B{}

　　3. Parameter Description

　　　　3.1, : It indicates the conditions under which the instruction is executed, which can be omitted

　　　　3.2, : The target address of the instruction jump. The instruction calculates the target address by the following method:

　　　　　　A. Extend the 24-bit signed complement immediate value to 32 bits

　　　　　　B. Shift the expanded 32-bit immediate value left by two bits

　　　　　　C. Add the obtained value to the PC register to get the jump target address

3. Jump instruction BL with connection

　　1. Function

　　　　 The jump instruction with connection BL copies the address of the next instruction to the r14 (i.e. return address connection register LR) register, and then jumps to the specified address to run the program.

　　2. Syntax format

　　　　 BL{}  

　　3. Parameter description is the same as instruction B

4. Jump instruction BX with state switching

　　1. Function

　　　　Make the program jump to the address specified by the parameter Rm specified in the instruction to execute the program

　　2. Syntax format

　　　　BX{}

　　3. Parameter Description

　　　　 contains the target address of the jump instruction,

5. Connection jump instruction BLX with state switching

　　1. Function

　　　　Used to make the program jump to Thumb state or return from Thumb state. This instruction is an unconditional execution instruction, and uses the lowest bit of the branch register to update the T bit in CPSR and write the return address to the link register LR.

　　2. Syntax format

　　　　 BLX{} 

　　3. Parameter Description

　　　　 is the jump target address of the instruction, which is calculated according to the following rules

　　　　A. Sign-extend the 24-bit offset specified by the instruction to form a 32-bit immediate value

　　　　B. Shift the result left by two places

　　　　C. Add bit H (bit [24]) to the first bit of the result address (bit [1])

　　　　D. Add the result to the program counter PC