Microprocessor 8085 Machine Cycle
Microprocessor 8085
Q: Why, in 8085, Opcode fetch have 4 T-States whereas all other machine cycles have only 3 T-States?
A: When an instruction is about to be executed, the first machine cycle that is performed by 8085 microprocessor is the opcode fetch machine cycle. Before knowing the opcode fetch machine cycle, we have to know what is actually the opcode is. There are 256 different instructions in 8085 microprocessor. Some of them are near about similar. To distinguish between all the 256 instructions, there are 256 different hexadecimal operational codes that is stored in the 8085 memory. Let an instruction be MVI A,23H is about to be executed. The hexadecimal code for MVI A is 3AH and that is fixed inside the 8085 memory. When the instruction as about to be executed, the programmer or the user will have to give input the 3AH data in the program input. When 8085 reads the input from the user, it first checks whether the input code is actually the opcode /operational code or any user defined data. Generally, the first input that is provided by the user is considered as the operational code for the specific instruction. So, after taking the input form the user, 8085 will jump to the memory where the operational codes are stored. From the memory, 8085 then fetch the operational code and come back to the execution process.
This is performed
by the opcode fetch machine cycle. The opcode fetch machine cycle is depicted
below:
Memory Location Mnemonics Opcode
8000H MVI
A,23H 3A H
8001H
23 H
The table for Machine cycle is given below:
|
Machine Cycle
|
Status of
|
Control Signals
|
Required
T-States
|
||
|
|
|
|
|||
|
Opcode Fetch(OF)
|
0
|
1
|
1
|
|
4
|
|
Memory Read(MR)
|
0
|
1
|
0
|
|
3
|
|
Memory Write(MW)
|
0
|
0
|
1
|
|
3
|
|
I/O Read(IOR)
|
1
|
1
|
0
|
|
3
|
|
I/O Write(IOW)
|
1
|
0
|
1
|
|
3
|
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