Instruction Format

 Instruction Format

An instruction is a command to the microprocessor to perform a given task on a specified data. Each instruction has two parts: one is task to be performed, called the operation code (opcode), and the second is the data to be operated on, called the operand. The operand (or data) can be specified in various ways. It may include 8-bit (or 16-bit ) data, an internal register, a memory location, or 8-bit (or 16-bit) address. In some instructions, the operand is implicit.
Instruction word size
The 8085 instruction set is classified into the following three groups according to word size:
1.      One-word or 1-byte instructions
2.      Two-word or 2-byte instructions
3.      Three-word or 3-byte instructions
In the 8085, "byte" and "word" are synonymous because it is an 8-bit microprocessor. However, instructions are commonly referred to in terms of bytes rather than words.
One-Byte Instructions
A 1-byte instruction includes the opcode and operand in the same byte. Operand(s) are internal register and are coded into the instruction. For example:

Task
Opcode
Operand
Binary Code
Hex Code
Copy the contents of the accumulator in the register C.
MOV
C,A
0100 1111
4FH
Add the contents of register B to the contents of the accumulator.
ADD
B
1000 0000
80H
Invert (compliment) each bit in the accumulator.
CMA

0010 1111
2FH

These instructions are 1-byte instructions performing three different tasks. In the first instruction, both operand registers are specified. In the second instruction, the operand B is specified and the accumulator is assumed. Similarly, in the third instruction, the accumulator is assumed to be the implicit operand. These instructions are stored in 8- bit binary format in memory; each requires one memory location.
MOV rd, rs
rd <-- rs copies contents of rs into rd.
Coded as 01 ddd sss where ddd is a code for one of the 7 general registers which is the destination of the data, sss is the code of the source register.
Example: MOV A,B
Coded as 01111000 = 78H = 170 octal (octal was used extensively in instruction design of such processors).
ADD r
A <-- A + r
Two-Byte Instructions
In a two-byte instruction, the first byte specifies the operation code and the second byte specifies the operand. Source operand is a data byte immediately following the opcode. For example:

Task
Opcode
Operand
Binary Code
Hex Code

Load an 8-bit data byte in the accumulator.
MVI
A, Data
0011 1110
DATA
3E
Data
First Byte
Second Byte







Assume that the data byte is 32H. The assembly language instruction is written as

Mnemonics
Hex code
MVI A, 32H
3E 32H1

The instruction would require two memory locations to store in memory.
MVI r,data
r <-- data
Example: MVI A,30H coded as 3EH 30H as two contiguous bytes. This is an example of immediate addressing.
ADI data
A <-- A + data
OUT port
where port is an 8-bit device address. (Port) <-- A. Since the byte is not the data but points directly to where it is located this is called direct addressing.
Three-Byte Instructions
In a three-byte instruction, the first byte specifies the opcode, and the following two bytes specify the 16-bit address. Note that the second byte is the low-order address and the third byte is the high-order address.
opcode + data byte + data byte
For example:

Task
Opcode
Operand
Binary Code
Hex Code

Transfer the program sequence to the memory location 2085H.

JMP
2085H
1100 0011
1000 0101
0010 0000
C3
85
20
First Byte
Second Byte
Third Byte








This instruction would require three memory locations to store in memory.
Three byte instructions - opcode + data byte + data byte
LXI rp, data16
rp is one of the pairs of registers BC, DE, HL used as 16-bit registers. The two data bytes are 16-bit data in L H order of significance.
rp <-- data16
Example:
LXI H,0520H coded as 21H 20H 50H in three bytes. This is also immediate addressing.
LDA addr
A <-- (addr) Addr is a 16-bit address in L H order. Example: LDA 2134H coded as 3AH 34H 21H. This is also an example of direct addressing.


REFERENCES
  1. R. S. Gaonkar, Microprocessor Architecture, Programming, and Applications with the 8085, Fifth Edition, Penram International Publishing (India) Private Limited.
  2. S Ghoshal, Microprocessor Based System Design, Macmillan India Limited, 1996
  3. M. Mano, Digital Logic and Computer Design, Prentice – Hall India
  4. B. Ram - Fundamentals of Microprocessor and Microcontrollers
  5. “Microprocessors: Principles and Applications” by A Pal
  6. “Microprocessors and Microcontrollers : Architecture, Programming and Interfacing Using 8085, 8086 and 8051” by Soumitra Kumar Mandal
  7. “Introduction to Microprocessors and Microcontrollers” by Crisp John Crisp
  8. “Microprocessors And Microcontrollers” by A Nagoor Kani
  9. “Microprocessors And Microcontrollers : Architecture, Programming and System Design 8085, 8086, 8051, 8096” by KRISHNA KANT
  10. “8 - Bit Microprocessor” by Vibhute

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