Data Transfer Instructions :
The MOV instruction is used to transfer a byte or a word of data from a source operand to a destination operand. These operands can be internal registers of the 8086 and storage locations in memory.
MOV instruction cannot transfer data directly between a source and a destination that both reside in external memory.
INPUT/OUTPUT INSTRUCTIONS :
IN acc, port : In transfers a byte or a word from input port to the AL register or the AX register respectively. The port number my be specified either with an immediate byte constant, allowing access to ports numbered 0 through 255 or with a number previously placed in the DX register allowing variable access (by changing the value in DX) to ports numbered from 0 through 65,535.
OUT port, acc : Out transfers a byte or a word from the AL register or the AX register respectively to an output port. The port numbers may be specified either with an immediate byte or with a number previously placed in the register DX allowing variable access.
No flags are affected.
XLAT (translate):
This instruction is useful for translating characters from one code such as ASCII to another such as EBCDIC, this is no operand instruction and is called an instruction with implied addressing mode.
The instruction loads AL with the contents of a 20 bit physical address computed from DS, BX and AL. This instruction can be used to read the elements in a table where BX can be loaded with a 16 bit value to point to the starting address (offset from DS) and AL can be loaded with the element number (0 being the first element number) no flags are affected.
XLAT instruction is equivalent to
MOV AL, [AL] [BX]
AL ← [(AL) + (BX) + (DS)]
Example :
Write a program to convert binary to gray code for the numbers 0 to F using translate instruction.
Let the binary number is stored at 0350 and its equivalent gray code is stored at 0351 after the program execution. Look up table is as follows.
The first two instructions LAHF and SAHF can be used either to read the flags or to change them respectively notice that the data transfer that takes place is always between the AH register and flag register. For instance, we may want to start an operation with certain flags set or reset. Assume that we want to preset all flags to logic 1. To do this we can first load AH with FF and then execute the SAHF instruction.
Example : Write an instruction sequence to save the contents of the 8086’s flags in memory location MEM1 and then reload the flags with the contents of memory location MEM2. Assume that MEM1 and MEM2 are in the same data segment defined by the current contents of DS.
Strings and String Handling Instructions :
The 8086 microprocessor is equipped with special instructions to handle string operations. By string we mean a series of data words or bytes that reside in consecutive memory locations. The string instructions of the 8086 permit a programmer to implement operations such as to move data from one block of memory to a block elsewhere in memory. A second type of operation that is easily performed is to scan a string and data elements stored in memory looking for a specific value. Other examples are to compare the elements and two strings together in order to determine whether they are the same or different.
Move String : MOV SB, MOV SW:
An element of the string specified by the source index (SI) register with respect to the current data segment (DS) register is moved to the location specified by the destination index (DI) register with respect to the current extra segment (ES) register.
The move can be performed on a byte (MOV SB) or a word (MOV SW) of data. After the move is complete, the contents of both SI & DI are automatically incremented or decremented by 1 for a byte move and by 2 for a word move. Address pointers SI and DI increment or decrement depends on how the direction flag DF is set.
Example : Block move program using the move string instruction
Load and store strings : (LOD SB/LOD SW and STO SB/STO SW)
LOD SB: Loads a byte from a string in memory into AL. The address in SI is used relative to DS to determine the address of the memory location of the string element.
(AL) ← [(DS) + (SI)]
(SI) ← (SI) + 1
LOD SW : The word string element at the physical address derived from DS and SI is to be loaded into AX. SI is automatically incremented by 2.
(AX) ← [(DS) + (SI)]
(SI) ← (SI) + 2
STO SB : Stores a byte from AL into a string location in memory. This time the contents of ES and DI are used to form the address of the storage location in memory
Repeat String : REP
The basic string operations must be repeated to process arrays of data. This is done by inserting a repeat prefix before the instruction that is to be repeated.
Prefix REP causes the basic string operation to be repeated until the contents of register CX become equal to zero. Each time the instruction is executed, it causes CX
to be tested for zero, if CX is found to be nonzero it is decremented by 1 and the basic string operation is repeated.
Example : Clearing a block of memory by repeating STOSB
The prefixes REPE and REPZ stand for same function. They are meant for use with the CMPS and SCAS instructions. With REPE/REPZ the basic compare or scan operation
can be repeated as long as both the contents of CX are not equal to zero and zero flag is 1.
REPNE and REPNZ works similarly to REPE/REPZ except that now the operation is repeated as long as CX≠0 and ZF=0. Comparison or scanning is to be performed as long as the string elements are unequal (ZF=0) and the end of the string is not yet found (CX≠0).
Moves a block of 32 consecutive bytes from the block of memory locations starting at offset address MASTER with respect to the current data segment (DS) to a block of locations starting at offset address copy with respect to the current extra segment (ES).
Auto Indexing for String Instructions :
SI & DI addresses are either automatically incremented or decremented based on the setting of the direction flag DF.
When CLD (Clear Direction Flag) is executed DF=0 permits auto increment by 1. When STD (Set Direction Flag) is executed DF=1 permits auto decrement by 1.
1. LDS Instruction:
LDS register, memory (Loads register and DS with words from memory)
This instruction copies a word from two memory locations into the register specified in the instruction. It then copies a word from the next two memory locations into the DS register. LDS is useful for pointing SI and DS at the start of the string before using one of the string instructions. LDS affects no flags.
Example 1 :LDS BX [1234]
Copy contents of memory at displacement 1234 in DS to BL. Contents of 1235H to BH. Copy contents at displacement of 1236H and 1237H is DS to DS register.
2. LEA Instruction :
Load Effective Address (LEA register, source)
This instruction determines the offset of the variable or memory location named as the source and puts this offset in the indicated 16 bit register.
