Timing diagram of INX H in 8085

In 8085 Instruction set, INX H stands for “INcrement eXtended register” and H stands for H-L register pair. This instruction will be used to add 1 to the present content of the H-L pair. And thus the result of the incremented content will remain stored in H-L pair itself. Though it is an arithmetic instruction but flag bits are not at all affected by the execution of this instruction. A register pair is generally used to store 16-bit memory address. If flag bits got affected during increment of a memory address, then it may cause problems in many cases. So as per design of 8085, flag bits are not getting affected by the execution of this instruction INX H.

INX H

Instruction INX H is a one-byte instruction, does not require data from memory or store data into memory. It occupies only 1-Byte in memory and only opcode fetch cycle is required to execute this instruction.

Mnemonics OperandOpcode (in HEX)Bytes
INX H231
INX H

INX B

Let us consider INX B as a sample instruction falling in this category. As it is a 1-Byte instruction, it will occupy single byte location in the memory. Let us consider that the initial content of register pair BC is 4050H. So after execution of the instruction INX B, the new content of BC register pair would be 4051H. The result of execution of this instruction is shown below with the help of a timing diagram.

Register PairBeforeAfter
(BC)4050H4051H
Address(H) Hex CodesMnemonicComment
200303INX BBC = BC + 1
INX B – execution report

Here we are considering that the hex code (Machine code) 03H, for the instruction INX B, is stored in memory location 2003H. To fetch this hex cod, the higher order and lower order address is given by the program counter and then the program counter is incremented by one to point to next instruction. The opcode fetch of INX need more time for decoding the opcode and internal operations so T5 and T6-states are required. Where as for other one byte general instruction, opcode fetch takes only T1 to T4-states.

Timing Diagram

Here is the timing diagram of the instruction execution INX B as below:

Fig 1: Timing diagram of the instruction INX B.

Fig 1: Timing diagram of the instruction INX B.
  • During T1, ALE remains high, and the content over AD0-AD7 will be the Low-Order memory byte (Here it is 03H). Similarly the content over A8-A15 will be the High order memory byte (Here it is 20H) and it remains for T1 to T3-states.
  • During T2 and T3 the ALE signal becomes LOW and remains low before starting of the next machine cycle. AD0-AD7 is now acted as data bus D0-D7. The RD’ (Active low read control signal) signal goes low during the T2 and T3-states and the processor now read the hex code(03H)from the memory location 2003H.
  • As the processor fetches opcode from memory, so IO/M’ signal remains Low during the total execution period to indicate that it is a memory related operation.
  • Two status lines S0 and S1 will be 1,1 to indicate that microprocessor is busy to fetch the opcode.
  • The opcode fetch of INX need more time for decoding the opcode and internal operations, so T4, T5 and T6-states are required.

Summary

So this instruction INX B requires 1-Byte, 1-Machine Cycle (Opcode Fetch) and 6 T-States for execution as shown in the timing diagram.

Similarly INX H also requires 6 T-States to execute it. Only notable difference is in machine code for the instruction INX H, which will be 23 H.

Fig 2: Timing diagram of the instruction INX H.
Fig 2: Timing diagram of the instruction INX H.

Author of this post

This post is co-authored by Professor Saraswati Saha, who is an assistant professor at RCCIIT, a renowned degree engineering college in India. Professor Saha teaches subjects related to digital electronics & microprocessors.

Timing diagram of INX H in 8085
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