Store instructions move data from registers to memory. Move instructions move data from one register to another. What is MOVS instruction in arm? The MOV instruction never affects the flags. Whenever the destination operand equals Zero, the Zero flag is set. The LDR pseudo-instruction is used for two main purposes: to generate literal constants when an immediate value cannot be moved into a register because it is out of range of the MOV and MVN instructions.
The mov instruction copies the data item referred to by its second operand i. While register-to-register moves are possible, direct memory-to-memory moves are not. Generally, LDR is used to load something from memory into a register, and STR is used to store something from a register to a memory address. The MOV instruction moves data bytes between the two specified operands.
The byte specified by the second operand is copied to the location specified by the first operand. The source data byte is not affected. The move pseudo instruction moves the contents of one register into another register.
This form can only be used inside an IT block. Rd and Rm must both be Lo registers. In ARMv6 and above, this restriction does not apply. Related concepts 4.
Related reference Above the parameters on the stack and below the base pointer , the call instruction placed the return address, thus leading to an extra 4 bytes of offset from the base pointer to the first parameter.
When the ret instruction is used to return from the subroutine, it will jump to the return address stored on the stack. Caller Rules To make a subrouting call, the caller should: Before calling a subroutine, the caller should save the contents of certain registers that are designated caller-saved. Since the called subroutine is allowed to modify these registers, if the caller relies on their values after the subroutine returns, the caller must push the values in these registers onto the stack so they can be restore after the subroutine returns.
To pass parameters to the subroutine, push them onto the stack before the call. The parameters should be pushed in inverted order i. Since the stack grows down, the first parameter will be stored at the lowest address this inversion of parameters was historically used to allow functions to be passed a variable number of parameters.
To call the subroutine, use the call instruction. This instruction places the return address on top of the parameters on the stack, and branches to the subroutine code. This invokes the subroutine, which should follow the callee rules below. After the subroutine returns immediately following the call instruction , the caller can expect to find the return value of the subroutine in the register EAX. To restore the machine state, the caller should: Remove the parameters from stack.
This restores the stack to its state before the call was performed. The caller can assume that no other registers were modified by the subroutine. Example The code below shows a function call that follows the caller rules. First parameter is in EAX, the second parameter is the constant ; the third parameter is in memory location var.
Thus, to get rid of the parameters, we can simply add 12 to the stack pointer. If the caller uses them after the call, it would have needed to save them on the stack before the call and restore them after it. Callee Rules The definition of the subroutine should adhere to the following rules at the beginning of the subroutine: Push the value of EBP onto the stack, and then copy the value of ESP into EBP using the following instructions: push ebp mov ebp, esp This initial action maintains the base pointer , EBP.
The base pointer is used by convention as a point of reference for finding parameters and local variables on the stack. When a subroutine is executing, the base pointer holds a copy of the stack pointer value from when the subroutine started executing. Parameters and local variables will always be located at known, constant offsets away from the base pointer value. We push the old base pointer value at the beginning of the subroutine so that we can later restore the appropriate base pointer value for the caller when the subroutine returns.
Remember, the caller is not expecting the subroutine to change the value of the base pointer. We then move the stack pointer into EBP to obtain our point of reference for accessing parameters and local variables.
Source contains either the data to be delivered immediate addressing or the address in register or memory of the data. Generally, the source data remains unaltered after the operation. In this addressing mode, a register contains the operand. Depending upon the instruction, the register may be the first operand, the second operand or both. As processing data between registers does not involve memory, it provides fastest processing of data.
An immediate operand has a constant value or an expression. When an instruction with two operands uses immediate addressing, the first operand may be a register or memory location, and the second operand is an immediate constant.
The first operand defines the length of the data. When operands are specified in memory addressing mode, direct access to main memory, usually to the data segment, is required.
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