In a previous article, I described a simple state machine to explain the prefetch behavior and miscellaneous observed bus delays of the Intel 8088. I

Adventures in PC Emulation

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2024-02-10 07:30:02

In a previous article, I described a simple state machine to explain the prefetch behavior and miscellaneous observed bus delays of the Intel 8088. I now believe that the model I described there - although successful in modelling the 8088's behavior - was unnecessarily complex. There are in fact far fewer legitimate bus "delays", and they do not relate to internal state transitions. I have also slightly revised my description of the BIU's prefetching logic - I now believe that the prefetching decision happens at the end of T2, instead of the beginning of T3. There may not be much of a distinction between the end of one cycle and the beginning of the next, but it lines up better in our new diagrams.

If you were interested enough to click on this article, you are probably already familiar with the 8088's bus cycle timings. A single bus cycle on the 8088 takes a minimum of 4 CPU cycles, consisting of 4 T-states numbered T1, T2, T3 and T4. Wait states (Tw) can be inserted between T3 and T4 via manipulation of the CPU's READY line to accommodate access to slower devices, but in general, the 4 T-states have been the demonstrable model of the 8088 for decades.

Diagrams such as the above have appeared in innumerable manuals, web references and computer science textbooks. It is no surprise then that emulator authors attempted to construct the logic and timings of their emulators around a 4-cycle bus cycle time. Unfortunately, this is an incomplete picture. For whatever reason, Intel was averse to advertising the true bus cycle time of the 8088 in their official documentation, and so this incomplete model has persisted to this day.