Problem – Draw the timing diagram of the given instruction in 8085,
MOV B, C
Given instruction copies the contents of the source register into the destination register and the contents of the source register are not altered.
MOV B, C Opcode: MOV Operand: B and C
Bis the destination register and C is the source register whose contents need to be transferred to the destination register.
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 opcode is stored, i.e., 00
- 20 – higher bit of address where opcode is stored, i.e., 20.
- ALE – provides signal for multiplexed address and data bus. Only in t1 it used as address bus to fetch lower bit of address otherwise it will be used as data bus.
- RD (low active) – signal is 1 in t1 & t4 as no data is read by microprocessor. Signal is 0 in t2 & t3 because here the data is read by microprocessor.
- WR (low active) – signal is 1 throughout, no data is written by microprocessor.
- IO/M (low active) – signal is 1 in throughout because the operation is performing on memory.
- S0 and S1 – both are 1 in case of opcode fetching.