Chapter 6 - 8086 instruction description and assembler directives from Microprocessors and Interfacing by Douglas Hall

Outline Instruction Description Assembler Directives

Instruction Description(contd.) AAA – ASCII Adjust for Addition 8086 allows you to add ASCII codes for two decimal digits without masking of the “3” in upper nibble of each. After the addition, the AAA instruction is used to make sure that the result is the correct unpacked BCD. The AAA instruction works only on AL register. AAD – BCD to binary convert before Division Converts two unpacked BCD digits in AH and AL to the equivalent binary number in AL. After the Division the AL will contain the unpacked BCD quotient and AH will contain the unpacked BCD remainder.

Instruction Description(contd.) AAM – BCD Adjust for Multiply After the two BCD digits are multiplied the AAM instruction is used to adjust the product to two unpacked BCD digits in AX. Works only on an operand in AL. Updates PF, SF and ZF. AAS – ASCII Adjust for Subtraction 8086 allows you to subtract ASCII codes for two decimal digits without masking of the “3” in upper nibble of each. After the subtraction, the AAA instruction is used to make sure that the result is the correct unpacked BCD. The AAS instruction leaves the correct unpacked BCD result in the low nibble of AL.

Instruction Description(contd.) ADC – Add with Carry, ADD – Addition ADC source, destination: adds the number from source to the number in destination and then stores the result in destination and the carry in CF. ADD source, destination: Does similar things as the ADC but do not store the carry in CF. Rather it discards the carry. AND AND destination, source: ANDs each bit in the source byte or word with each bit in the destination byte or word. The result is put into specified destination register/location, the contents of the source does not change.

Instruction Description(contd.) CALL – Call a procedure Used to transfer the execution to another subprogram or procedure. There are two basic types: NEAR: a call to a procedure which is in the same code segment as the CALL instruction. FAR: a call to a procedure which is in different segment then the CALL instruction. CBW – Convert signed Byte to signed Word Copies the sign of a byte in AL to all the bits in the AH. AH is then said to be the sign extension of the AL. The CBW must be done before a signed byte in AL can be divided by another signed byte with the IDIV instruction.

Instruction Description(contd.) CLC, CLD, CLI, CMC CLC: Instruction resets the carry flag to 0. CLD: resets the direction flag to 0. CLI: resets the interrupt flag to 0. CMC: complement the carry flag i.e. from 01 and from 10. CMP – Compare byte or word CMP destination, source: this instruction compares the byte from the source to the byte in the destination or a word from the specified source to the word from the specified destination. Flags AF, OF, SF, ZF, PF and CF flags are updated to show the result of the comparison.

Instruction Description(contd.) CMPS/CMPSB/CMPSW – Compare String Bytes or String Words Used to compare a byte in one string with the byte in another string or a word in one string with the word in another string. SI is used to hold offset of source string and DI is used for the destination string. CWD – Convert signed Word to signed Double Word Copies the sign bit of a word in AX to all the bits in the DX register. It must be done before the signed word in AX is divisible by another signed word with the IDIV instruction. It affects no flags. DAA, DAS DAA: used to make sure the result of adding two BCD number is a BCD number. DAS: used after subtracting two BCD numbers to make sure the result is correct packed BCD.

Instruction Description(contd.) DEC – Decrement Destination register or memory DEC destination: subtracts one from the destination word or byte. AF, OF, PF, SF and ZF flags are updated. DIV – Unsigned Divide Used to divide unsigned word by a byte or to divide an unsigned double word by a word. The word must be present in AX register and divisor must be in register or memory location. After division AL contains the 8-bit result (quotient) and AH will contain the 8-bit remainder.

Instruction Description(contd.) ESC – Escape Used to pass instructions to the coprocessor such as 8087 math processor shares the address and data bus with an 8086. HLT – Halt processing It causes 8086 to stop fetching and executing instructions. Processor will be in the halt state. The only way it can be resume is by generating interrupt at the INTR pin. IDIV – Divide by signed byte or word IDIV source: the instruction is used to divide a signed word by a signed byte, or to divide signed double word by a word. The operand must be in AX the divisor may be in register or memory location. The final quotient is stored in AL and remainder will be stored in AH.

Instruction Description(contd.) IMUL – Multiply signed numbers IMUL source: multiplies a signed byte from some source times a signed byte in AL or a signed word from some source times a signed word in AX. CF and OF flags are updated. IN – copy data from a port IN Accumulator, port: copy data from a port to the AL or AX register. If 8-bit port is read then the data will be read into AL else if 16-bit port is read the data will go into AX. INC – Increment INC Destination: it adds one to the source word or byte specified in the destination. AF, OF, PF, SF and ZF are affected.

Instruction Description(contd.) INT – Interrupt program execution INT Type: type refers to any number between 0 to 255. When 8086 executes the INT instruction it does following: Decrement the stack pointer by 2 and PUSH the flags onto the stack. Decrement the stack pointer by 2 and PUSH the contents of CS onto the stack. Decrement the stack pointer by 2 and PUSH the offset of the next instruction after INT into the stack. Get a new value for IP from an absolute memory address of 4 times the type specified in the instruction. Get a new value for CS from an absolute memory address of 4 times the type specified in the instruction plus 2. Reset both IF and TF.

Instruction Description(contd.) INTO – Interrupt on overflow Decrement the stack pointer by 2 and PUSH the flags onto the stack. Decrement the stack pointer by 2 and PUSH the contents of CS onto the stack. Decrement the stack pointer by 2 and PUSH the offset of the next instruction after INTO onto the stack. Reset TF and IF. IRET – Interrupt Return Use at the end of the interrupt service procedure to return execution to the interrupted program. It reloads the contents of the stack and changes the IP value.

Instruction Description(contd.) JA/JNBE – Jump if Above/Jump in Not Below or Equal After compare or some other instruction the instruction will cause the execution to jump to a label given in the instruction. No flags are affected. JAE/JNB/JNC – Jump if Above or Equal/Jump if Not Below/Jump in No Carry If after a compare or some other instruction the CF is 0 this instruction will cause the execution to jump on the specified label. If CF is 1 the instruction will have no effect. JB/JC/JNAE – Jump if Below/Jump if Carry/Jump if Not Above or Equal If after a compare or some other instruction the CF is 1 this instruction will cause the execution to jump on the specified label. If CF is 0 the instruction will have no effect.

Instruction Description(contd.) JBE/JNA – Jump if Below or Equal/Jump if Not Above If after a compare or some other instruction the CF is 1 this instruction will cause the execution to jump on the specified label. If CF and ZF both are 0 the instruction will have no effect. JCXZ – Jump if the CX register is 0 JE/JZ – Jump if Equal/Jump if Zero JG/JNLE – Jump if Greater/Jump if Not Less than or Equal After compare the instruction will cause a jump if ZF is 0 and CF is same as the OF. The destination must be in the range of -128 bytes to +127 bytes. JL/JNGE – Jump if Less than/Jump if Not Greater than or Equal After compare instruction it will cause the jump if SF is not equal to OF.

Instruction Description(contd.) JLE/JNG – Jump if Less than or Equal/Jump if Not Greater After compare instruction it will cause the jump if ZF is set or SF is not equal to OF. The destination must be in the range of -128 bytes to +127 bytes. JMP – Unconditional Jump to the Specified Location It will always force the 8086 to jump to the specified location. The more similar instructions are JNE – Jump if Not equal, JNZ – Jump if Not Zero, JNO – Jump if No Overflow, JNP – Jump if No Parity, JPO – Jump if Odd Parity, JNS – Jump if Not Signed, JO – Jump if Overflow, JP – Jump if Parity, JPE – Jump if Parity Even, JS – Jump if Signed etc.

Instruction Description(contd.) LAHF – Copy low byte of Flag register to AH Lower byte of the 8086 is same as the flag bytes of the 8085. Thus it copies the flag bytes of the 8085 to the AH register. LDS – Load register and DS with the word from memory LDS – Register, memory address of first word: it copies the word from two memory locations into the register specified in the instruction. It then copies the word from the next two memory location into DS register. LEA – Load Effective Address LEA register, source: determines offset of the variable or memory location named source and puts it into 16-bit register.

Instruction Description(contd.) LES – Load register and ES with the word from memory This instruction loads new values into the specified register and ES register from four successive memory locations. The word from the first two memory location will be copied to the specified register and the word from the next two locations will be copied in to the ES register. LOCK – Assert Bus lock Signal Allows microprocessors to ensure that the other processor can’t take control of the system bus while it is in critical instruction which uses the system bus.

Instruction Description(contd.) LODS/LODSB/LODSW – Load String word or byte into AL Copies the byte from a string location pointed to by SI to AL or a word to AX. LOOP – Jump to specified Label if CX! =0 After Auto decrement Used to repeat a series of instruction a number of times. The number of time the sequence is repeated is stored in CX register Variants are LOOPE, LOOPZ, LOOPNE and LOOPNZ. MOV – Copy a word or byte MOV destination, source: copies a word or byte from the source byte to the destination byte. The variants are MOVS/MOVSB/MOVSW – move string, byte or move string word.

Instruction Description(contd.) MUL – Multiply unsigned bytes or words MUL source: multiplies the unsigned byte from source times an unsigned byte in the AL register or word in the AX register. NEG – Forms 2’s complement NEG Destination: replaces the number in destination with the 2’s complement of that number NOP – NO Operation It simply uses the 3 clock cycles and increments the instruction pointer to point to the next instruction.  OR – Logically OR OR Destination, Source: ORs each bit in source byte or word with the bit in destination byte or word. The result is put in the specified destination.

Instruction Description(contd.) OUT – Output a Byte or word to a port OUT Port, Accumulator: the Out instruction copies a byte from AL or a word from AX to the specified port. Two types of ports: Fixed and Variable port POP POP destination: copies the word from the stack top location to the destination specified. The stack pointer is automatically incremented by 2. PUSH PUSH Source: decrements the stack pointer by 2 and copies the word specified in the source to the location pointed by the stack pointer. PUSHF: push the flag register onto the stack.

Instruction Description(contd.) RCL – Rotate operand around to the left through CL Rotates all the bits in the specified byte or word to some bits in left. It is circular because the MSB of the operand is stored in to the CF and then CF is stored in to the LSB of the operand. Similarly RCR: rotates the operand through right using CF. REP/REPE/REPNE/REPZ/REPNZ – Repeat String Instruction until specified condition exists REP is the prefix which will cause the CX register to be decremented and the string to be repeated until CX=0. REPE is repeat is equal, REPZ is repeat if zero causes the string to be repeated as long as the words are equal and ZF=1 respectively. RET – Return Execution from the procedure call Return execution from the procedure to the next instruction after the CALL instruction. It do this by restoring the stack contents to the registers and setting IP to point to the next instruction after the CALL instruction.

Instruction Description(contd.) ROL – rotate Left, ROR – Rotate Right ROL Destination, count: rotate the bits specified in destination to count no of bit positions. Similarly ROR do the same thin but it rotates towards right. SAHF- Copy AH register to low byte of Flag register The lower byte of the 8086 flag register exactly corresponds to the 8085 flag register. This instruction replaces this 8085 flag register with the byte from AH register. SAL/SHL – Shift operand bit in left, put zero in LSBs, SAR – shift right SAL/SHL Destination, count: shifts each bits in the specified destination to some number of bits specified by the count value towards left. SAR destination, count: shifts in right.

Instruction Description(contd.) SBB – Subtract with Borrow, SUB - subtract SUB Destination, source: subtracts the number in source from the number in destination and the final result is stored in destination. It ignores borrow. SBB: it does the same thing but do not ignore borrow. SCAS/SCASB/SCASW – Scans a String or Byte or Word Compares a byte in AL or a word in AH with a byte or word pointed to by DI in ES. Therefore the string to be compared must be in extra segment and the DI must contain the offset of the byte or word. STC/STD/STI – set carry flag, set direction flag, set interrupt flag to 1. STOS/STOSB/STOSW – Stores byte or word in string Copies a byte from AL or a word from AX to the memory location in extra segment pointed by DI. After copy DI is automatically incremented or decremented to point to the next string element in memory.

Instruction Description(contd.) TEST – AND operands to Update ANDs the contents of the source byte with the contents of the destination byte. Flags are updated but neither operand is changed. WAIT – Wait for test signal or interrupt signal 8086 enters in idle condition and does no processing. It will stay in the idle position until 8086 TEST input pin is made low or interrupt signal is received on INTR or NMI pins. XCHG XCHG destination, source: changes the contents of destination with contents of source and vice versa. Destination and Source both must be either byte or word. It can be register or memory location.

Instruction Description XLAT/XLATB – Translate a byte in AL Used to translate a byte from one code to another code. Replaces the byte in AL register with the byte pointed to by BX in a lookup table in the memory. The lookup table must be in the memory before execution of XLAT. XOR – eXclusive OR XOR Destination, Source: instruction exclusive ORs each bit in the source byte or word with the bit in the destination byte or word. The result will follow the truth table of XOR. The answer will be stored in the destination.

Assembler Directives(contd.) Assume Used to tell the assembler the name of the logical segment it should use for a specified segment. You must tell the assembler that what to assume for any segment you use in the program. Example, ASSUME: CODE tells the assembler that the instructions for the program are in segment named CODE. DB – Defined Byte Used to declare a byte type variable or to set aside one or more locations of type byte in memory. Example, PRICES DB 49H, 98H, 29H: Declare array of 3 bytes named PRICES and initialize 3 bytes as shown.

Assembler Directives(contd.) DD – Define Double Word Used to declare a variable of type doubleword or to reserve a memory location which can be accessed as doubleword. DQ – Define Quadword Used to tell the assembler to declare the variable as 4 words of storage in memory. DT – Define Ten bytes Used to tell the assembler to declare the variable which is 10 bytes in length or reserve 10 bytes of storage in memory. DW – Define Word Used to tell the assembler to define a variable type as word or reserve word in memory.

Assembler Directives(contd.) END – End the program To tell the assembler to stop fetching the instruction and end the program execution. ENDP – it is used to end the procedure. ENDS – used to end the segment. EQU – Equate Used to give name to some value or symbol. EVEN – Align on Even memory address Tells the assembler to increment the location counter to the next even address if it is not already at an even address.

Assembler Directives(contd.) EXTRN Used to tell the assembler that the name or labels following the directive are in some other assembly module. GLOBAL – Declares symbols as PUBLIC or EXTRN Used to make the symbol available to other modules. It can be used in place of EXTRN or PUBLIC keyword. GROUP – Group related segment Used to tell the assembler to group the logical segments named after the directive into one logical segment. This allows the content of all the segments to be accessed from the same group.

Assembler Directives(contd.) INCLUDE – include source code from file Used to tell the assembler to insert a block of source code from the named file into the current source module. This shortens the source code. LABEL Used to give the name to the current value in the location counter. The LABEL directive must be followed by a term which specifies the type you want associated with that name. LENGTH Used to determine the number of items in some data such as string or array.

Assembler Directives(contd.) NAME Used to give a specific name to a module when the programs consisting of several modules. OFFSET It is an operator which tells the assembler to determine the offset or displacement of named data item or procedure from the start of the segment which contains it. ORG – Originate Tells the assembler to set the location counter value. Example, ORG 7000H sets the location counter value to point to 7000H location in memory. $ is often used to symbolically represent the value of the location counter. It is used with ORG to tell the assembler to change the location according to the current value in the location counter. E.g. ORG $+100.

Assembler Directives(contd.) PROC – Procedure Used to identify the start of the procedure. PTR – Pointer Used to assign a specific type to a variable or a label. It is necessary to do this in any instruction where the type of the operand is not clear. Public It is used to tell the assembler that the specified label or variable is accessible by other modules. This is useful in large programs which are generally written in modules.

Assembler Directives SEGMENT Used to indicate that the start of a logical segment. Preceding the segment directive is the name you want to give to the segment. SHORT Used to tell the assembler that only a 1-byte displacement is needed to code a jump instruction. If the jump destination is after the jump instruction in the program, the assembler will automatically reserve 2 bytes for the displacement. TYPE Tells the assembler to determine the type of a specified variable. The TYPE operator can be used in instruction such as ADD BX, TYPE WORD_ARRAY, where we want to increment BX to point to the next word in an array of words.