The jump instruction is used to realize the jump of the program flow. There are two ways to realize the jump of the program flow in the ARM program:
(1) Use special jump instructions.
(2) Write the jump address value directly to the program counter PC.
By writing the jump address value to the program counter PC, any jump in the 4GB address space can be realized, which can be used in combination before the jump
MOV LR, PC
For similar instructions, you can save the next instruction address as the future return address value, so as to realize the subroutine call in the 4GB continuous linear address space. Dedicated jump instruction
B, BL, BX, BLX and BXJ:
Jump, jump with link (jump with return), jump and switch instruction set, jump with link and switch instruction set (jump with return and switch instruction set), jump and switch to Jazelle state.
1. B command
The format of the B command is:
B{condition} destination address
The B instruction is the simplest jump instruction. Once a B instruction is encountered, the ARM processor will immediately jump to the given target address and continue from there.
Continued execution. Note that the actual value stored in the jump instruction is an offset from the current PC value, not an absolute address, and its value is calculated by the assembler (refer to relative addressing in addressing mode). It is a 24-bit signed number, which is signed to 32 bits after shifting to the left by two bits. The effective offset is 26 bits (32MB before and after the address space). The following instructions:
B Label; the program jumps to the label Label unconditionally for execution
CMP R1, # 0; when the Z condition code in the CPSR register is set, the program jumps to the label Label for execution
BEQ Label 2, BL instruction
The format of the BL instruction is:
BL{condition} target address
BL is another jump instruction, but before the jump, the current content of the PC will be saved in the register R14. Therefore, the content of R14 can be reloaded into the PC to return to the instruction after the jump instruction. . This instruction is a basic but commonly used means to implement subroutine calls. The following instructions:
BL Label; when the program unconditionally jumps to the label Label for execution, the current PC value will be saved to R14 at the same time. 3. BLX instruction
The format of the BLX instruction is:
BLX destination address
The BLX instruction jumps from the ARM instruction set to the target address specified in the instruction, and switches the working state of the processor from the ARM state to the Thumb state. The instruction also saves the current content of the PC to the register R14. Therefore, when the subroutine uses the Thumb instruction set and the caller uses the ARM instruction set, the BLX instruction can be used to implement the call of the subroutine and the switch of the processor working state.
At the same time, the return of the subroutine can be completed by copying the value of register R14 to the PC.
4. BX instruction The format of BX instruction is:
BX{condition} target address
The BX instruction jumps to the target address specified in the instruction. The instruction at the target address can be either an ARM instruction or a Thumb instruction. to sum up
grammar
op1{cond}{.W} <wbr />label
op2{cond} <wbr />Rm
among them:
op1
Is one of the following:
B
Jump.
BL
Jump with link
BLX
Jump with link and switch instruction set.
op2
Is one of the following:
BX
Jump and switch instruction set.
BLX
Jump with link and switch instruction set.
BXJ
Jump and switch to Jazelle execution.
cond
Is an optional condition code. cond cannot be used in all forms of this instruction.
.W
Is an optional instruction width specifier used to force the use of 32-bit B instructions in Thumb-2.
label
Is a program-relative expression.
Rm
It is a register that contains the target address to jump to.
operating
All these instructions will cause a jump, or jump to label, or jump to the address contained in Rm. In addition:
The BL and BLX instructions can copy the address of the next instruction to lr (r14, link register).
The BX and BLX instructions can change the state of the processor from ARM to Thumb, or from Thumb to ARM.
BLX label always changes the state of the processor in any case.
BX Rm and BLX Rm can calculate the target state from bit [0] of Rm:
If bit [0] of Rm is 0, the state of the processor will change to (or remain in) the ARM state
If bit [0] of Rm is 1, the state of the processor will change to (or remain in) the Thumb state.
The BXJ instruction will change the state of the processor to Jazelle
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http://luleimi.blog.163.com/blog/static/175219645201210922139272/
