Timing diagram of MOV Instruction in Microprocessor

Problem – Draw the timing diagram of the given instruction in 8085,

MOV B, C 

Given instruction copy the contents of the source register into the destination register and the contents of the source register are not altered.

Example:

MOV B, C
Opcode: MOV 
Operand: B and C 

Bis is the destination register and C is the source register whose contents need to be transferred to the destination register. Algorithm – The instruction MOV B, C is of 1 byte; therefore the complete instruction will be stored in a single memory address. For example:

2000: MOV B, C 

Only opcode fetching is required for this instruction and thus we need 4 T states for the timing diagram. For the opcode fetch the IO/M (low active) = 0, S1 = 1 and S0 = 1. The timing diagram of MOV instruction is shown below:  

In Opcode fetch ( t1-t4 T states):

• 00 – lower bit of address where the opcode is stored, i.e., 00.
• 20 – higher bit of address where the opcode is stored, i.e., 20.
• ALE – provides signal for multiplexed address and data bus. Only in t1 is it used as an address bus to fetch a lower bit of address otherwise it will be used as the data bus.
• RD (low active) – signal is 1 in t1 & t4 as no data is read by the microprocessor. Signal is 0 in t2 & t3 because here the data is read by a microprocessor.
• WR (low active) – signal is 1 throughout, no data is written by a microprocessor.
• IO/M (low active) – signal is 1 throughout because the operation is performing on memory.
• S0 and S1 - both are 1 in case of opcode fetching.