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Instruction System - Arithmetic Instruction

Instruction System - Arithmetic Instruction. 计算机学院 李征 Tel ： 13882153765 Email ： lizheng@cs.scu.edu.cn OICQ: 1340915. Arithmetic Instruction. (1) ADD (2) ADC (3) INC (4) SUB (5) SBB (6) DEC (7) NEG (8) CMP. Arithmetic Instruction.

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Instruction System - Arithmetic Instruction

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  1. Instruction System -Arithmetic Instruction 计算机学院 李征 Tel：13882153765 Email：lizheng@cs.scu.edu.cn OICQ: 1340915

  2. Arithmetic Instruction • (1) ADD • (2) ADC • (3) INC • (4) SUB • (5) SBB • (6) DEC • (7) NEG • (8) CMP

  3. Arithmetic Instruction • Arithmetic instruction represent operation status in flags. • All arithmetic instructions affect status flags.

  4. (1) ADD (Addition) • ADD DEST， SRC • DEST <=（SRC）+（DEST） • Corresponding status flag: • OF、SF、ZF、AF、PF、CF

  5. (1) ADD (Addition) • Example: • ADD BX，SI • ADD DA_WORD,0F8CH • ADD DL,TAB[BX]

  6. ALU + 目的地址 源地址

  7. (1) ADD (Addition) • Example: • MOV AH，45H • MOV AL，0E3H • ADD AH，AL • 45H = 01000101B • 0E3H =11100011B • Unsigned code: 69+227 (Decimal) • Signed code: 69+（-29）(Decimal)

  8. (1) ADD (Addition) • 01000101 • + 11100011 • 1 00101000 • Unsigned code：Carry, CF=1 • BCD code: No carry, AF=0 • Signed code：positive+negative OF=0 • Result is positive, SF=0 • Non-zero result，ZF=0 • Bit number of ‘1’ is even，PF=1

  9. (2) ADC (Addition with carry) • ADC DEST， SRC • DEST <=（SRC）+（DEST）+（CF） • Corresponding status flag: • OF、SF、ZF、AF、PF、CF

  10. (2) ADC (Addition with carry) • Example: addition of two 32-bit codes, one code is stored in AX and DX. • ADD AX，1A23H • ADC DX，76FH

  11. ALU + 目的地址 源地址

  12. (2) ADC (Addition with carry) • Example: • MOV AL，93H • MOV AH，02H • MOV BL，88H • MOV BH，0EEH • ADD AL，BL • ADC AH，BH

  13. (2) ADC (Addition with carry) • Explain for unsigned code: • （AX）= 0293H 659（Decimal） • （BX）=0EE88H 61064（Decimal） • ADC（高） ADD（低） • 00000010 10010011 • + 11101110 10001000 • 0 11110001 1 00011011 • CF CF • Result：F11BH 61723（Decimal） This CF represents if there is overflow. This CF joins the addition operations

  14. (2) ADC (Addition with carry) • Explain for signed code: • （AX）= 0293H 659（Decimal） • （BX）=0EE88H - 4472（Decimal） • ADC（高） ADD（低） • 00000010 10010011 • + 11101110 10001000 • 0 11110001 1 00011011 • CF CF • OF=0 OF=1 • Result：F11BH -3813（Decimal） This CF is invalid. This CF joins the addition operations. This OF represent if there is overflow. This OF is invalid.

  15. (3) INC (Increase) • INC DEST • DEST <=（DEST）+1 • Corresponding status: • OF、SF、ZF、AF、PF • Why not CF?

  16. ALU +1 目的地址 1

  17. (3) INC (Increase) • Example: addition of data with 5 word length • mov bx, 0 • mov cx, 5 • clc • lop1: mov ax, tab1[bx] • adc ax, tab2[bx] ; generate CF • mov res[bx], ax • inc bx ; not affect CF • inc bx • loop lop1 ; not affect flags

  18. (3) INC (Increase) • Example: • MOV AL，0FFH • INC AL • CF = ? Maintain its status before ‘INC’, may be 1 or 0

  19. (4) SUB (Subtraction) • SUB DEST，SRC • DEST <=（DEST）-（SRC） • Corresponding status flag: • OF、SF、ZF、AF、PF、CF

  20. ALU - 目的地址 源地址

  21. (4) SUB (Subtraction) • Note: • Do not use signed code operation for CF judgment. • In CPU, CF generation is not correspond to the subtraction operation itself.

  22. (5) SBB (Subtraction with borrow) • SBB DEST，SRC • DEST<=（DEST）-（SRC）-（CF） • Corresponding status flag: • OF、SF、ZF、AF、PF、CF

  23. ADC and SBB • With ADC and SBB, long code operation can be realized in CPU. • Basically, they are both based on CF. • Operation order: lower part first, then higher part

  24. (6) DEC (Decrease) • DEC DEST • DEST <=（DEST）-1 • Corresponding status flag: • OF、SF、ZF、AF、PF • not affect CF

  25. (7) NEG (Negative) • NEG DEST • DEST <= -（DEST） • Corresponding status flag: • OF、SF、ZF、AF、PF、CF

  26. NEG ALU 0 目的地址

  27. (7) NEG (Negative) • Generally, the operation data is explained as signed code. • If operation code is unsigned, then there must be overflow (except 0). • CF = 1 is always the case, except operation code is 0.

  28. (7) NEG (Negative) • Overflow in signed codes: • -128 in byte, -32768 in word, etc.

  29. (7) NEG (Negative) • NEG is limited by code length. • Example: Negative the 32-bit code in DAW • NEG DAW • MOV AX，0 • SBB AX，DAW+2 • MOV DAW+2，AX

  30. (8) CMP (Compare) • CMP DEST，SRC • （DEST）-（SRC） • Corresponding status flag: • OF、SF、ZF、AF、PF、CF • Same as SUB, but not store operation result

  31. ALU - 目的地址 源地址

  32. (8) CMP (Compare) • Not store operation result, only store the status • Along with program transfer instructions, it can realize some branch structure.

  33. (8) CMP (Compare) • Explain for unsigned compare: • CF=0：（DEST）≥（SRC） • CF=1：（DEST）<（SRC）

  34. (8) CMP (Compare) • Explain for signed compare: • OF=SF：（DEST）≥ （SRC） • 1) OF=SF=0: • OF=0 -> SF=0 is correct • 2) OF=SF=1: • OF=1 -> SF=1 is incorrect, so SF=0 is correct.

  35. (8) CMP (Compare) • OF≠SF：（DEST）<（SRC） • 1) OF=0，SF=1: • OF=0 -> SF=1 is correct • 2) OF=1，SF=0: • OF=1 -> SF=0 is incorrect, so SF=1 is correct.

  36. (8) CMP (Compare) • The explaining of unsigned or signed compare is decided by different program transfer instruction choice. • If there is ZF in judgment, there will be more compare result.

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