Instructions format of 8085

ROHINI COLLEGE OF ENGINEERING & TECHNOLOGY

2.1 INSTRUCTION FORMAT OF 8085

The 8085 have 74 basic instructions and 246 total instructions. The instruction set of 8085

is defined by the manufacturer Intel Corporation. Each instruction of 8085 has 1 byte

opcode. With 8 bit binary code, we can generate 256 different binary codes. In this, 246

codes have been used for opcodes.




Figure 2 .1.1 Instruction Format o f 8085

[Source: Microprocessor
“ Architecture Programming and”Application
byR.S. Gaonkar
, page
-131]
The size of 8085 instructions can be 1 byte, 2 bytes or 3 bytes.

 The 1-byte instruction has an opcode alone.

 The 2 bytes instruction has an opcode followed by an eight-bit address or data.

 The 3 bytes instruction has an opcode followed by 16 bit address or data. While

storing the 3 bytes instruction in memory, the sequence of storage is, opcode first

followed by low byte of address or data and then high byte of address or data.

ADDRESSING MODES

Every instruction of a program has to operate on a data. The method of specifying

the data to be operated by the instruction is called Addressing. The 8085 has the following

5 different types of addressing.

1. Immediate Addressing

2. Direct Addressing

3. Register Addressing

4. Register Indirect Addressing

5. Implied Addressing