Microprocessor & assembly language
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This document discusses the PUSH, POP, PUSHF, and POPF instructions in x86 assembly. It explains that PUSH decrements the stack pointer and copies data onto the stack, while POP increments the stack pointer and copies data off the stack. PUSHF pushes the flags register onto the stack, while POPF pops the flags register off the stack. It provides examples of using these instructions and diagrams illustrating how the stack and stack pointer are affected.
Microprocessor & assembly language
- 2. Presentation Topic : PUSH, PUSHF, POP, POPF Presented By :
- 3. Contents : The Stack Push Pop PushF PopF
- 4. A stack in one-dimensional data structure. Items are added and removed from one end of the structure; that is, it is processed in a “Last-in, first out” manner. The most recent addition to the stack is called the top of the stack. A program must set aside a block of memory to hold the stack. We have been dong this by declaring a stack segment ; For example : .STACK 100H THE STACK
- 5. To add a new word to the stack we PUSH it on. The syntax is PUSH source Where source is a 16-bit register or memory word. For example, PUSH AX Execution of PUSH causes the following to happen : 1. SP is decreased by 2. 2. A copy of the source content is moved to the address specified by SS:SP. The source is unchanged. PUSH AND PUSHF
- 6. The instruction PUSHF, which has no operands, pushes the contents of the FLAGS register onto the stack. Initially SP contains the offset address of the memory location immediately following the stack segment; the first PUSH decreases SP by 2, making it point to the last word in the stack segment. Because each PUSH decreases SP, the stack grows toward the beginning of memory PUSH AND PUSHF
- 7. FIGURE : EMPTY STACK offset 00F0 00F2 00FA 00F4 00F6 00F8 00FC 00FE 0100 1234 SP 5678 0100 AX BX STACK(empty) SP
- 8. FIGURE : AFTER PUSH AX offset 00F0 00F2 00FA 00F4 00F6 00F8 00FC 00FE 0100 1234 1234 SP 5678 00FE AX BX STACK SP
- 9. FIGURE : AFTER PUSH BX offset 00F0 00F2 00FA 00F4 00F6 00F8 00FC 00FE 0100 5678 1234 1234 SP 5678 00FC AX BX STACK SP
- 10. To remove the top item from the stack we POP it. The Syntax is POP destination Where destination is a 16-bit register (except IP) or memory word. For example, POP BX Executing POP causes this to happen: 1. The content of SS:SP (the top the stack) is moved to the destination 2. SP is increased by 2. POP AND POPF
- 11. The instruction POPF pops the top of the stack into the FLAGS Register. There is no effect of PUSH, PUSHF, POP, POPF on the flags. Such as Illegal : PUSH DL Is illegal. So is a push of immediate data, such as Illegal : PUSH 2
- 12. FIGURE : BEFORE POP offset 00F0 00F2 00FA 00F4 00F6 00F8 00FC 00FE 0100 5678 1234 FFFF SP 0001 00FC CX DX STACk SP
- 13. FIGURE : AFTER POP CX offset 00F0 00F2 00FA 00F4 00F6 00F8 00FC 00FE 0100 5678 1234 5678 SP 0001 00FE CX BX STACK SP
- 14. FIGURE : AFTER POP DX offset 00F0 00F2 00FA 00F4 00F6 00F8 00FC 00FE 0100 5678 1234 5678 SP 1234 0100 CX DX STACK(empty) SP
- 15. THANK YOU END