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Got rid of runState, implemented a solution for WAIT

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Jeff 2016-09-29 11:04:33 -07:00 • committed by Jeff Parsons
commit b3ba03d0a5
7 changed files with 358 additions and 338 deletions
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23
modules/pdp11/lib/cpuops.js
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@ -1097,7 +1097,6 @@ PDP11.opHALT = function(opCode)
this.regErr |= PDP11.CPUERR.BADHALT;
this.trap(PDP11.TRAP.BUS_ERROR, PDP11.REASON.HALT);
} else {
this.runState = 3;
this.endBurst();
}
this.nStepCycles -= 1;
@ -1605,7 +1604,27 @@ PDP11.opTSTB = function(opCode)
*/
PDP11.opWAIT = function(opCode)
{
this.checkInterruptDelay();
/*
* The original PDP-11 emulation code would actually stop emulating instructions now, relying on assorted
* setTimeout() callbacks, setInterval() callbacks, device XHR (XMLHttpRequest) callbacks, etc, to eventually
* call interrupt(), which would then transition the CPU's runState from 2 to 0 and kickstart emulate() again.
*
* That approach isn't compatible with PCjs emulators, which prefer to rely on the simulated CPU clock to
* drive all simulated device updates. This means components should call the CPU's setTimer() function, which
* invokes the provided callback when the number of CPU cycles that correspond to the requested number of
* milliseconds have elapsed. This also gives us the ability to scale device response times as needed if the
* user decides to crank up CPU speed, and to freeze them along with the CPU whenever the user halts the machine.
*
* However, the PCjs approach requires the CPU to continue running. One simple solution to this dilemma:
*
* 1) opWAIT() sets a new opFlags bit (OPFLAG.WAIT)
* 2) When stepCPU() sees OPFLAG.WAIT, it checks for interrupts; if none, it rewinds the PC back to the WAIT
*
* NOTE: It's almost always a bad idea to add more checks to the inner stepCPU() loop, because every additional
* check can have a measurable (negative) impact on performance. Which is why it's important to use opFlags bits
* whenever possible, since we can test for multiple (up to 32) exceptional conditions with a single check.
*/
this.opFlags |= PDP11.OPFLAG.WAIT;
this.nStepCycles -= 1;
};