Comparison between 8086 and 8088

All the changes in 8088 above 8086 are indirectly or directly related to the 8-bit, 8085 compatible data and control bus interface.

1) The predecoded code queue length is reduced to 4 bytes in 8088, whereas the 8086 queue have 6 bytes. This was done to get ride from the unnecessary prefetch operations and optimize the use of the bus by BIU when prefetching the instructions.

2) The 8088 bus interface unit will fetch a byte from memory to load the queue each time, if at least one byte is free. In case of 8086, at least two bytes should be free for the next fetch operation.

3) The whole execution time of the instructions in 8088 is affected by the 8-bit external data bus. Now all the16-bit operations require additional 4 clock cycles. The CPU speed is also restricted by the speed of instruction fetches.

The pin assignments  of both the CPUs are closely identical, however, they have the following functional changes.

1.) A8-A15 already latched, every time valid address bus.

2. ) BHE has no meaning as the data bus is only of 8-bits.

3.) SS0 provides the S0 status information in  theminimum mode.

4.) IO/M has been inverted to be compatible with 8085 bus structure