LOGICAL Group of Instructions.
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Bit manipulation instruction
Bit Manipulation instructions : The instruction in this category operate on bits. The instruction can set, reset or even can test a particular bit. These instructions include :
Logical instructions: NOT, AND, OR, XOR and Test.
| FORMAT | Example | Comments | Flags Affected |
| NOT Source (R/M) | MOV BL, 35h NOT BL |
BL=00110101 BL= 11001010 |
|
| AND (R/M), Imm./ R/M | MOV AL, 35h AND AL, 0Fh |
AL=00110101 AL=AL.0Fh= 00000101 |
CF=OF=0, AF=? PF, SF, ZF updated |
| OR (R/M), Imm./ R/M | MOV BL, 22h MOV CL, FFh OR BL, CL |
BL=00010110 CL=11111111 BL=BL+CL=11111111 |
CF=OF=0, AF=? PF, SF, ZF updated |
| XOR (R/M), Imm./ R/M | MOV AL, 55h XOR AL, FAh |
AL=01010101 AL xor FAh=10100111 |
CF=OF=0, AF=? PF, SF, ZF updated |
| TEST R?M, Imme/R/M | MOV AL, 51h TEST AL, 80h |
AL=01010001 Test AL,80 test if the MSB is 0 or not; if msb is zer0 Z flag will be set. |
CF=OF=0, AF=? PF, SF, ZF updated |
2. Shift and Rotate operation
These instructions are used to perform logical or arithmetic shift and rotate operations where a bit of the data is involved. These instructions in shift group are: SHL, SAR, SHR, SAR
| FORMAT | Description | Example | Comments | Flags Affected |
| SHL dest, count | Shift logical left byte or word; No. of time shift count in CL | MOV CL, 02 MOV BX, 89h SHL BX, CL |
CL= 00000010 BX=00000000 10001001 BX= 0000001000100100 |
SHL inserts zeros from the right |
| SAL dest, count | Shift arithmetic left byte or word; No. of time shift count in CL | MOV CL, 02 MOV BX, 89h SHL BX, CL |
CL= 00000010 BL=01001001 BL=00100100, CF= 1 |
SHL inserts zeros from the right, and MSB remains unaffected and the MSB is put in CF |
| SHR dest, count | Shift logical right byte or word; No. of time shift count in CL | MOV CL, 02 MOV BX, 89h SHL BX, CL |
CL= 00000010 BX= 10001001 BX= 00100010 |
Bits are shifted right with LSB moved in CF and zeros inserted from MSB |
| SAR dest, count | Shift arithmetic right byte or word; No. of time shift count in CL | MOV CL, 02 MOV BX, 89h SHL BX, CL |
CL= 00000010 BL= 10001001 BL= 00100100 |
MSB remains same i.e is copied again in MSB, i.e sign of number remains the same |
3. Rotate instructions: ROL, ROR, RCL, RCR
ROL : Rotate left byte or word MSB to LSB and to Carry Flag [CF]. The operand to be rotated can be a register or memory location.
| FORMAT | Example | Comments | Flags Affected |
| ROL Dest, count | MOV AL, 35 ROL AL, 03 |
AL 00100101 CF=1, AL=00101001 |
CF, OF |
- ROR : Rotate right byte or word LSB to MSB and to Carry Flag [CF]. The operand to be rotated can be a register or memory location.
| FORMAT | Example | Comments | Flags Affected |
| ROR Dest, count | MOV AL, 35 ROR AL, 03 |
AL 00100101 CF=1, AL=10100100 |
CF, OF |
- RCL : Rotate left byte or word through carry flag MSB TO CF and CF to LSB4. The operand to be rotated can be a register or memory location.
| FORMAT | Example | Comments | Flags Affected |
| RCL Dest, count | MOV AL, 35 RCL, AL 03 |
AL 00100101 CF=1, AL=00101001 |
CF, OF |
- RCR : Rotate right byte or word through carry flag LSB TO CF and CF to MSB. The operand to be rotated can be a register or memory location.
| FORMAT | Example | Comments | Flags Affected |
| RCR Dest, count | MOV AL, 35 ROL AL, 03 |
Initially assume CF=0 AL 00100101 CF=0, AL=01000101 |
CF, OF |
Example:
Write an ALP to convert the 8-bit packed BCD number stored in the memory into a binary number and store it in another memory location
| Using assemble such as MASM | Using KIT |
| Source variable X
Destination variable : Y |
Assume source packed data at location : 2500:1000
Converted Binary to be stored at 2500: 1001 |
| Data SEGMENT
X db 35h Y db ? Data ENDS Code SEGMENT ASSUME CS: code, DS: data MOV ax, data MOV ds, ax MOV AL, X MOV BL, AL AND AL, 0F0h MOV CL, 04 ROR AL, CL MOV BH, 0Ah MUL BH AND BL, 0Fh ADD AL, BL MOV Y, AL Code ENDS END
|
MOV ax, 2500
MOV ds, ax MOV AL, [1000] MOV BL, AL AND AL, 0F0h MOV CL, 04 ROR AL, CL MOV BH, 0Ah MUL BH AND BL, 0Fh ADD AL, BL MOV [1001], AL HLT |