8086 Programming. Multiplication Instructions. Multiplication can be performed on signed and unsigned numbers.

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1 Multiplication Instructions 8086 Programming Multiplication can be performed on signed and unsigned numbers. MUL IMUL source source x AL source x AX source AX DX AX The source operand can be a memory location or a register. Immediate operand is not allowed. For signed multiplication, the product is in true binary form, if positive, and in two's complement form, if negative. These are the same forms used to store all positive and negative signed numbers used by the microprocessor. Upon completion, the Carry Flag (CF) and the Overflow Flag (OF) tell how much of the product is relevant. For MUL, if the high-order half of the product is zero, CF and OF are 0; otherwise, they are 1. 1

2 For IMUL, if the highorder half of the product is just a signextension of the low-order half, CF and OF are 0; otherwise, they are 1. By checking these flags it can be determined whether or not the magnitude of the product was small enough to fit into a single byte (word). The remaining condition flags may be changed, but their contents are undefined, i.e., they are unpredictable. Assembly Language Operation MUL CL IMUL DH IMUL BYTE PTR[BX] MUL TEMP MUL CX IMUL DI MUL WORD PTR[SI] ; the signed product is in AX ; the signed product is in AX ; the unsigned product is in AX ; the unsigned product is in DX-AX ; the signed product is in DX-AX ; the unsigned product is in DX-AX 2

3 Suppose that BL and CL each contain two 8-bit unsigned numbers, and these numbers must be multiplied to form a 16-bit product stored in DX. This procedure cannot be accomplished by a single instruction because we can only multiply a number times the AL register for an 8-bit multiplication. Example shows a short program that generates DX = BL x CL. EXAMPLE MOV BL,5 ;load data MOV CL,l0 MOV AL,CL ;position data MUL BL ;multiply MOV DX,AX ;position product For the signed multiply instruction, IMUL, the sign of the product is determined by the normal sign rules of algebra. The unsigned multiply instruction, MUL, treats the operands as unsigned numbers and produces an unsigned product. To contrast the IMUL and MUL instructions, suppose that the (AL) = E4, which is as an 8-bit signed integer and as an unsigned integer, and the (EL) = 11, which is both as an 8-bit signed integer and as an unsigned integer. Then IMUL BLwould cause (AX) = FAF4 = and (CF) = (OF) = 1 (which implies that the result cannot be put in 8 bit). The instruction MUL BL would cause (AX) = 0BF4 = and (CF) = (OF) = 1 (which implies that the result cannot be put in 8 bit). 3

4 Division Instructions Division can be performed on signed and unsigned numbers. DIV source IDIV source quotient AL remainder AH source AX quotient remainder source AX DX DX AX 4

5 The source operand can be a memory location or a register. Immediate operand is not allowed. 5

6 Logic Instructions AND instruction Uses any addressing mode except memory to memory and segment registers addressing. Flags affected: S, Z and P. C = 0. AND dest,source ; dest dest source AND CX,DX ;CX CX DX AND AL,BL AND CL,33h AND ARRAY[SI],AL The AND operation is used for masking x xxxxxxx x xxx OR instruction OR AL,BH AL AL BH OR SI,DX SI SI DX OR BX,[SI] OR DATA[SI+20h],AL XOR instruction XOR AL,BH AL AL BH XOR SI,DX SI SI DX XOR BX,[SI] XOR DATA[SI+20h],AL 6

7 The XOR operation is used for. x xxxxxxx XOR AL,AL AL OR CX,0600h ; set bits. and. AND CX,0FFFCh ; clear bits. and. XOR CX,1000h ;invert bit 7

8 Test and bit test Instructions TEST instructionperformes the AND operation without changing the destination. TEST dest,cource ; dest source and update flags TEST CX,DX ;CX & DX TEST AL,BL TEST CL,33h TEST ARRAY[SI],AL RIGHT: LEFT: TEST AL,1 ; test. JNZ RIGHT ;jump if set TEST AL,128d ;test. JNZ LEFT ;jump if set.... addbx,cx 8

9 TEST AL,1 ; test right bit JNZ RIGHT ;jump if set TEST AL,128 ;test the left bit JNZ LEFT ;jump if set 9

10 NOT and NEG NOT is a logical inversion NEG is an arithmetic inversion ( 2 s complement) NOT CH ; CH is one s complement NEG DL ;DL is two s complement NEG BX NOT TEMP NOT BYTE PTR [BX] 10

11 Shift and Rotate Instructions SAL/SHL SAR SHR dest,count dest,count dest,count OF is undefined if count 1. PF, SF and ZF flags are affected by shift instructions but left unchanged by the rotate instructions. The SAL instruction multiplies the signed binary number by a power of 2. The SAR instruction divides the signed binary number by a power of 2. CF Target register or memory SHL 0 CF SAL 0 SHR 0 CF SAR CF 11

12 Example: Using DEBUG or Emu8086, evaluate the results of the following: MOV AX,-9 MOV BL,2 IDIV BL MOV AX,-9 SAR AX,1 12

13 RCL dest,count Addressing Modes: All except immediate and segment registers. Count must be 1 or CL. CF Target register or memory RCL ROL CF RCR ROR CF 13

14 Problems: 1. Write a program that finds the number of 1s in abyte. Hint: use rotate instruction. 2. Write a program that finds the number of 1s in a word. 14