Gregory's ServerThe bus hold feature is annoyingly complicated to reverse engineer because the 8086 has two hardware modes: minimum and maximum. Bus hold is probably too complicated to explain in a Mastodon thread but let's see what happens.
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 In minimum mode, the device requests the bus through the HOLD signal, and the 8086 acknowledges it with HLDA. Three flip-flops manage the request, letting it proceed when the 8086 is ready.  @kenshirriff I guess 8237 must only use the HOLD/HLDA lines. I'm not sure I even knew about the RQ/GT lines. |
In minimum mode, the 8086's signals are similar to the older 8080 processor. This made it easier for customers to migrate. Maximum mode provides complex, encoded control signals that support more functionality for multiprocessor or coprocessor systems.