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ECE291 Computer Engineering II Lecture 4

ECE291 Computer Engineering II Lecture 4. Josh Potts University of Illinois at Urbana- Champaign. Outline. Logic instructions Shifting instructions Arithmetic operations Overflow and carries Important flags setting. SEGMENT code myvar1 DW 01234h ; define word variable ; (value=1234h)

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ECE291 Computer Engineering II Lecture 4

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  1. ECE291Computer Engineering IILecture 4 Josh Potts University of Illinois at Urbana- Champaign

  2. Outline • Logic instructions • Shifting instructions • Arithmetic operations • Overflow and carries • Important flags setting ECE291

  3. SEGMENT code myvar1 DW 01234h; define word variable ; (value=1234h) myvar2 DW 01234; define word variable ; (value=1234d = 4D2) myvar3 RESW ; define word variable ; (value uncertain) myvar4 DW 01BCDh ece291msg DB 'ECE291 is great' ..start: mov ax,cs ; set up data segment mov ds,ax ; DS=CS; any memory reference we make is; assumed to reside in the DS segment mov ax,[myvar2] ; AX <- [myvar2] ; == mov ax,[2] mov si,myvar2; use SI as a pointer to myvar2 ; (equiv C code: SI=&myvar2 ) mov ax,[si] ; read memory at myvar2 ; (*(&myvar2)) ; (indirect reference) mov bx,ece291msg; BX is a pointer to a string ; (equiv C code: ;BX=&ece291msg) dec BYTE [bx+1] ; make that 'C' a 'B' !!!! mov si, 1 ; Use SI as an index inc [ece291msg+SI]; == inc [SI + 8] ; == inc [9] Memory Access Example ECE291

  4. ;Memory can be addressed using four registers: ; SI -> Assumes DS ; DI -> Assumes DS ; BX -> Assumes DS ; BP -> Assumes SS !!! ; Examples: mov ax,[bx]; ax <- word in memory pointed to by BX mov al,[bx]; al <- byte in memory pointed to by BX mov ax,[si] ; ax <- word pointed to by SI mov ah,[si] ; ah <- byte pointed to by SI mov cx,[di] ; cx <- word pointed to by DI mov ax,[bp] ; ax <- [SS:BP]STACK OPERATION! ; In addition, BX+SI and BX+DI are allowed: mov ax,[bx+si] mov ch,[bx+di] ; Furthermore, a fixed 8-bit or 16-bit ; displacement from the registers!: mov ax,[23h] ; ax <- word at DS:0023 mov ah,[bx+5] ; ah <- byte at DS:(BX+5) mov ax,[bx+si+107] ; ax <- word at ; DS:(BX+SI+107) mov ax,[bx+di+47] ; ax <- word at ; DS:(BX+DI+47) ; REMEMBER: memory to memory moves are ; ILLEGAL!!! mov [bx],[si] ; ILLEGAL mov [di],[si] ; ILLEGAL (use movsw) ; Special case: stack operations! pop word [myvar] ; [myvar] <- SS:[SP] Memory Access Example (cont.) ECE291

  5. AC(Alignment check) (VM) Virtual mode (RF) Resume (NT) Nested task (IOPL) Input/output privilege level (O) Overflow (D) Direction (I) Interrupt (T) Trace (S) Sign (Z) Zero (A) Auxiliary Carry (P) Parity (C) Carry 8086, 8088, 80186 80386, 80486DX 80286 80486SX Flag Register ECE291

  6. Logic Instructions • Logic instructions operate on a bit-by-bit basis NOT: A =~A AND: A &= B OR: A |= B XOR: A ^= B • Except for NOT these instructions affect the flags as follows: • clear the carry (C) • clear the overflow (O) • set the zero flag (Z) if the result is zero, or clear it otherwise • copy the high order bit of the result into the sign flag (S) • set the parity bit (P) according to the parity (number of 1s) in the result • scramble the auxiliary carry flag (A) • The NOT instruction does not affect any flag ECE291

  7. Logic Instructions (cont.) • TEST (non-destructive AND) - logically ands two operands and sets the flags but does not save the result • typically one would use this instruction to see if a bit contains one e.g., test al, 1 • sets the flags same as AND instruction but doesn’t change al • AND and OR instructions are often used to mask out data • a mask value is used to force certain bits to zero or one within some other value • a mask typically affects certain bits and leaves other bits unaffected • AND forces selected bits to zero and cl, 0fh • OR forces selected bits to one or cl, 0fh ECE291

  8. Shifting Instructions ECE291

  9. Shifting Instructions (cont.) • SHL/SAL (shift left/shift arithmetic left) • moves each bit of the operand one bit position to the left the number of times specified by the count operand • zeros fill vacated positions at the L.O. bit; the H.O. bit shifts into the carry flag • A quick way to multiply by two • Useful in packing data, e.g., consider two nibbles in AL and AH that we want to combine shl ah, 4 ;requires 80286 or later or al, ah NOTE:There are two forms of shifts 1) use of immediate shift count (8086, 8088 allow an immediate shift of 1 only, e.g., shl ax, 1)2) use of register CL to hold the shift count ECE291

  10. Example ;multiply AX by decimal 10 (1010) (multiply by 2 and 8, then add results) shl ax, 1 ;AX times 2 mov bx, ax ;save 2*AX shl ax, 2 ;2*(original)AX * 4 = 8*(original)AX add ax, bx ;2*AX + 8*AX = 10*AX ;multiply AX by decimal 18 (10010) (multiply by 2 and 16, then add results) shl ax, 1 ;AX times 2 mov bx, ax ;save 2*AX shl ax, 3 ;2*(original)AX times 8 = 16*(original)AX add ax, bx ;2*AX + 16*AX = 18*AX ECE291

  11. Shifting Instructions (cont.) • SHR(shift right) • shifts all the bits in the destination operand to the right one bit • zeros fill vacated positions at the H.O. bit • the L.O. bit shifts into the carry flag • A quick way to divide by two (works for unsigned numbers) • Useful for unpacking data, e.g., suppose you want to extract the two nibbles in the ALregister, leaving the H.O. nibble in AH and the L.O. nibble in AL: mov ah, al ;get a copy of the H.O. nibble shr ah, 4 ;move H.O. to L.O. and clear H.O. and al, 0fh ;remove H.O. nibble from AL ECE291

  12. Shifting Instructions (cont.) • SAR (shift arithmetic right) • shifts all the bits in the destination operand to the right one bit replicating the H.O. bit • the L.O. bit shifts into the carry flag • Main purpose is to perform a signed division by some power of two e.g., mov ax, -15 sar ax, 1 ; Result is -8 • In 80286 and later you can use SAR to sign extend one register into another, e.g., mov ah, al sar ah, 7 If AL contains 11110001 then AH will contain sign bits extension 1111111111110001 ECE291

  13. Shifting Instructions (cont.) • RCL (rotate through carry left) • rotates bits to the left, through the carry flag • bit in the carry flag is written back into bit zero (on the right) • ROL (rotate left) • rotates bits to the left • shifts operand’s H.O. bit into bit zero (the L.O. bit) • e.g., extract bit 10 to 14 in AX and leave these bits in 0 to 4 rol ax, 6 and ax, 1fh NOTE:There are two forms of rotate 1) use of immediate rotate count (8086, 8088 allow an immediate rotate of 1 only, e.g., ROL AX, 1)2) use of register CL to hold the rotate count ECE291

  14. Shifting Instructions (cont.) • RCR(rotate through carry right) • rotates bits to the right, through the carry flag • bit in the carry flag is written back into H.O. bit (on the left) • ROR(rotate right) • rotates bits to right • shifts operand’s L.O. bit into H.O. bit ECE291

  15. Shifting OperationsExample mov ax,3 ; Initial register values AX = 0000 0000 0000 0011 mov bx,5 ; BX = 0000 0000 0000 0101 or ax,9 ; ax <- ax | 0000 1001 AX = 0000 0000 0000 1011 and ax,10101010b ; ax <- ax & 1010 1010 AX = 0000 0000 0000 1010 xor ax,0FFh ; ax <- ax ^ 1111 1111 AX = 0000 0000 1111 0101 neg ax ; ax <- (-ax) AX = 1111 1111 0000 1011 not ax ; ax <- (~ax) AX = 0000 0000 1111 0100 Or ax,1 ; ax <- ax | 0000 0001 AX = 0000 0000 1111 0101 shl ax,1 ; logical shift left by 1 bit AX = 0000 0001 1110 1010 shr ax,1 ; logical shift right by 1 bit AX = 0000 0000 1111 0101 ror ax,1 ; rotate left (LSB=MSB) AX = 1000 0000 0111 1010 rol ax,1 ; rotate right (MSB=LSB) AX = 0000 0000 1111 0101 mov cl,3 ; Use CL to shift 3 bits CL = 0000 0011 shr ax,cl ; Divide AX by 8 AX = 0000 0000 0001 1110 mov cl,3 ; Use CL to shift 3 bits CL = 0000 0011 shl bx,cl ; Multiply BX by 8 BX = 0000 0000 0010 1000 ECE291

  16. Simple Arithmetic Instructions • ADD(addition): A += B • Register addition, e.g., add ax, bx • Immediate addition, e.g., add dl, 33h • Memory to register addition, e.g., memory data added to AL: mov di, NUMB ;address of NUMB mov al, 0 ;clear sum add al, [di] ;add [NUMB] add al, [di + 1] ;add [NUMB + 1] • NOTE: any ADD instruction modifies the contents of the sign, zero, carry, auxiliary carry, parity, and overflow flags ECE291

  17. Simple Arithmetic Instructions (cont.) • INC (increment addition): A++ mov di, NUMB ;address of NUMB mov al, 0 ;clear sum add al, [di] ;add [NUMB] inc di ;di = di + 1 add al, [di] ;add [NUMB + 1] NOTE: The increment instruction does not affect the carry flag bit. ECE291

  18. Simple Arithmetic Instructions (cont.) • ADC (addition with carry) - functions as regular addition, except the bit in the carry flag (C) is also added to the result • used mainly to add numbers that are wider than 16 bits (8086 - 80286) or wider than 32 bits in the 80386, 80486, Pentium) • Example: • addition of two 32-bit numbers (BXAX) + (DXCX): add ax, cx adc bx, dx ECE291

  19. Simple Arithmetic Instructions (cont.) • SUB(subtraction): A -= B • Register subtraction, e.g., sub cl, bl • Immediate subtraction, e.g., mov ch, 22h sub ch, 34h • NOTE: any SUB instruction modifies the contents of the sign, zero, carry, auxiliary carry, parity, and overflow flags Result is -12 (1110 1110) Flags change: Z = 0 (result not zero) C = 1 (borrow) A = 1 (half-borrow) S = 1 (result negative) P = 0 (even parity) 0 = 0 (no overflow) ECE291

  20. Simple Arithmetic Instructions (cont.) • DEC (decrement subtraction): A--, subtracts a 1 from a register or the contents of a memory location e.g., DEC BH NOTE:The decrement instruction does not affect the carry flag bit. • SBB (subtract with borrow) functions as regular subtraction, except the carry flag (C), which holds the borrow, also subtracts from the difference • used mainly to subtract numbers that are wider than 16 bits (8086 - 80286) or wider than 32 bits (in the 80386, 80486, Pentium) • Example: • subtraction of two 32-bit numbers (BXAX) - (SIDI): sub ax, di sbb bx, si ECE291

  21. Overflow and Carries • Carry • indicates a carry after addition or a borrow after subtraction • CF: carry flag (unsigned) {1 = CY (there is carry); 0 = NC (no carry)} • e.g., 36,864 (9000h) + 36,864 (9000h) = 73,728 (12000h) > 65,535 (FFFFh) {OV, CY} • carry is set when unsignedgoes out of range (denotes an unsigned arithmetic overflow) • Overflow • condition that occurs when signed numbers are added or subtracted • OF: overflow flag (signed) {1 = OV, 0 = NV} • e.g., 20,480 (5000h) + 20,480 (5000h) = 40,960 (A000h) > 32,767 (7FFFh) {OV, NC} • overflow is set when signedgoes out of range (denotes a signed arithmetic overflow) • Example: FFFFh + FFFFh = FFFEh {(-1) + (-1)} = -2; NV, CY ECE291

  22. Overflows & Carries Example mov ax,0fffeh ; 65534 interpreted as unsigned add ax,3 ; C = 1, O = 0 --- Unsigned Overflow Condition mov ax,0FFFEh ; -2 interpreted as signed add ax,3 ; C = 1, O = 0 --- Okay as signed mov bx,07FFFh ; 32767 interpreted as signed add bx,1 ; C = 0, O = 1 --- Signed Overflow Condition mov bx,07FFFh ; 32767 interpreted as unsigned add bx,1 ; C = 0, O = 1 --- Okay as unsigned Mov ax,07h ; 7 interpreted as either signed or unsigned Add ax,03h ; C = 0, O = 0 --- Okay as either ECE291

  23. Flag Setting FLAG Name Description Notes ZF Zero 1:ZR:Zero 1 indicates that the result was zero 0:NZ: Non-zero CF Carry 1:CY Unsigned Math and shifting 0:NC Needed a carry or borrow OF Overflow 1:OV Signed Math 0:NV Also (+) or (-) to be represented as a valid two’s complement number SF Sign Flag 1:NG: - MSB of result 0:PL: + ECE291

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