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Chapter2c

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MaeEstherMaguadMaralit

The document discusses control transfer instructions and conditional jumps in assembly language programming. It describes the EFLAGS register which contains status flags like carry, parity, zero, and sign that are checked by conditional jump instructions. Conditional jumps allow altering the default sequential execution of instructions by jumping to other parts of the code based on the status flag values. The JMP instruction provides an unconditional jump by specifying a label to transfer execution to. JMP changes the instruction pointer EIP to point to the target label, altering the program flow.

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Structured Assembly Language Programming Techniques Control Transfer Instructions
More on Conditional Jumps  Instructions that check the eFLAGs register before jumping  The FLAGs checked by Conditional jumps – Carry – Parity – Zero – Sign – Overflow flags
The eFLAGs Register  A special purpose register  Certain bits in this register serve as Flags
The eFLAGs Register FLAGS SET (1) CLEARED (0) Overflow OV [overflow] NV [no overflow] Sign NG [negative] PL [positive] Zero ZR [zero] NZ [not zero] Parity PE [even] PO [odd] Carry CY [carry] NC [no carry]
Conditional Jumps
Sequential Statements  Fetch-Decode-Execute CS:EIP is the PC (Program Counter)  When the fetched instruction is copied into the instruction register, EIP is automatically incremented by X. X = instruction length (in bytes)  Since EIP is automatically INCREMENTED by X, the instructions are executed SEQUENTIALLY by default.
Recall CPU MEMORY address CU inst 1 07E2:0000 INPUT inst 2 07E2:0002 PC = ? inst 3 07E2:0004 OUTPUT IR = inst 1 ... 07E2:0006 ALU
How can we change the default execution?  What will happen to EIP? High-level code/algorithm: Current instruction ► if (condition) then do1 IP points here ► do1: code 0 code 1 code 2 else do2: code x code y code z
JMP Statement JMP means jump (an UNCONDITIONAL jump)  This instruction takes one operand: a label  For example: jmp doon LABEL dito_ba: mov eax, 4 mov ebx, 1 mov ecx, msg1 mov edx, len1 int 80h doon: mov eax, 1 mov ebx, 0 int 80h
JMP Statement nasm –f elf sample.asm –l sample.lst
JMP Statement machine code/ opcode of
JMP Statement machine code/ opcode of ?
JMP Statement jmp a value in backwords storage format; therefore actual value is: 00000016 So we have: E9 00000016
JMP Statement When Instruction Register contains E9 00000016 CS:EIP will ‘point’ here So EIP = 00000005 h Note: As the instruction in IR is executed: EIP will be: EIP+00000016 = 0000001B h
JMP Statement When Instruction Register contains E9 00000016 CS:EIP will ‘point’ here So EIP = 00000005 h Note: As the instruction in IR is executed: EIP will be: EIP+00000016 = 0000001B h
Addition of hex numbers Decimal: 19 + 9 28 Hexadecimal: 19 + 9 22
JMP Statement Reminder:  The displacement is in Backwords Storage Format.  A displacement in a jump can either be a positive or a negative value.
JMP Statement FFFF FFE5
JMP Statement The value is negative, so this jumps to a label ‘above’ FFFF FFE5 sign-bit F = 1111 (binary)
JMP Statement IR EIP The value is negative, so this jumps to a label ‘above’ FFFF FFE5 + 0000 001B

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Chapter2c

  • 1. Structured Assembly Language Programming Techniques Control Transfer Instructions
  • 2. More on Conditional Jumps  Instructions that check the eFLAGs register before jumping  The FLAGs checked by Conditional jumps – Carry – Parity – Zero – Sign – Overflow flags
  • 3. The eFLAGs Register  A special purpose register  Certain bits in this register serve as Flags
  • 4. The eFLAGs Register FLAGS SET (1) CLEARED (0) Overflow OV [overflow] NV [no overflow] Sign NG [negative] PL [positive] Zero ZR [zero] NZ [not zero] Parity PE [even] PO [odd] Carry CY [carry] NC [no carry]
  • 6. Sequential Statements  Fetch-Decode-Execute CS:EIP is the PC (Program Counter)  When the fetched instruction is copied into the instruction register, EIP is automatically incremented by X. X = instruction length (in bytes)  Since EIP is automatically INCREMENTED by X, the instructions are executed SEQUENTIALLY by default.
  • 7. Recall CPU MEMORY address CU inst 1 07E2:0000 INPUT inst 2 07E2:0002 PC = ? inst 3 07E2:0004 OUTPUT IR = inst 1 ... 07E2:0006 ALU
  • 8. How can we change the default execution?  What will happen to EIP? High-level code/algorithm: Current instruction â–º if (condition) then do1 IP points here â–º do1: code 0 code 1 code 2 else do2: code x code y code z
  • 9. JMP Statement JMP means jump (an UNCONDITIONAL jump)  This instruction takes one operand: a label  For example: jmp doon LABEL dito_ba: mov eax, 4 mov ebx, 1 mov ecx, msg1 mov edx, len1 int 80h doon: mov eax, 1 mov ebx, 0 int 80h
  • 10. JMP Statement nasm –f elf sample.asm –l sample.lst
  • 11. JMP Statement machine code/ opcode of
  • 12. JMP Statement machine code/ opcode of ?
  • 13. JMP Statement jmp a value in backwords storage format; therefore actual value is: 00000016 So we have: E9 00000016
  • 14. JMP Statement When Instruction Register contains E9 00000016 CS:EIP will ‘point’ here So EIP = 00000005 h Note: As the instruction in IR is executed: EIP will be: EIP+00000016 = 0000001B h
  • 15. JMP Statement When Instruction Register contains E9 00000016 CS:EIP will ‘point’ here So EIP = 00000005 h Note: As the instruction in IR is executed: EIP will be: EIP+00000016 = 0000001B h
  • 16. Addition of hex numbers Decimal: 19 + 9 28 Hexadecimal: 19 + 9 22
  • 17. JMP Statement Reminder:  The displacement is in Backwords Storage Format.  A displacement in a jump can either be a positive or a negative value.
  • 18. JMP Statement FFFF FFE5
  • 19. JMP Statement The value is negative, so this jumps to a label ‘above’ FFFF FFE5 sign-bit F = 1111 (binary)
  • 20. JMP Statement IR EIP The value is negative, so this jumps to a label ‘above’ FFFF FFE5 + 0000 001B