Timing diagram of MVI instruction

Problem – Draw the timing diagram of the following code,

MVI B, 45 

Explanation of the command – It stores the immediate 8 bit data to a register or memory location.

Example: MVI B, 45
Opcode: MVI
Operand: B is the destination register and 45 is the source data which needs to be transferred to the register.
’45’ data will be stored in the B register.

Algorithm –

• Decide what is the opcode and what is the data. Here, opcode is ‘MVI B’ and data is 45.
• Assume the memory address of the opcode and the data. For example:

  MVI B, 45
  2000: Opcode
  2001: 45

• The opcode fetch will be same in all the instructions.
• Only the read instruction of the opcode needs to be added in the successive T states.
• For the opcode fetch the IO/M (low active) = 0, S1 = 1 and S0 = 1. Also, 4 T states will be required to fetch the opcode from memory.
• For the opcode read the IO/M (low active) = 0, S1 = 1 and S0 = 0. Also, only 3 T states will be required to read data from memory.



In Opcode fetch ( t1-t4 T states ) –

• 00 – lower bit of address where opcode is stored.
• 20 – higher bit of address where opcode is stored.
• 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, no data is read by microprocessor. Signal is 0 in t2 & t3, data is read by microprocessor.
  WR (low active) – Signal is 1 throughout, no data is written by microprocessor.
  IO/M (low active) – Signal is 0 in throughout, operation is performing on memory.
  S0 and S1 – Signal is 1 in t1 to t4 states, as to fetch the opcode from the memory.

In Opcode read ( t5-t7 T states ) –

• 01 – lower bit of address where data is stored.
• 20 – higher bit of address where data is stored.
• ALE – Provides signal for multiplexed address and data bus. Only in t5 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 t5 as no data is read by microprocessor. Signal is 0 in t6 & t7 as data is read by microprocessor.
  WR (low active) – Signal is 1 throughout, no data is written by microprocessor.
  IO/M (low active) – Signal is 0 in throughout, operation is performing on memory.
  S0 – Signal is 0 in throughout, operation is performing on memory to read data 45.
  S1 – Signal is 1 throughout, operation is performing on memory to read data 45.