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Instruction history buffer tweaks

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This commit is contained in:
Jeff Parsons 2015-07-18 15:54:11 -07:00
commit da2f8ddda2
3 changed files with 101 additions and 88 deletions
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19
blog/2015/07/17/README.md
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@ -58,12 +58,14 @@ Then I ran into a couple of Windows 95 oddities. First, some kernel initializat
executed an invalid opcode (0x0F,0xFF) and expected its DPMI exception (0x06) handler to field the exception; that was
fixed by flagging the opcode as genuinely invalid.
By default, PCjs marks opcodes as invalid *only* if they are truly invalid. PCjs considers the vast
majority of unused/undocumented opcodes to be merely "undefined" until we've seen them in the wild. An instruction will
be marked invalid if we either discover that real hardware generates an Invalid Opcode exception *or* that real software
expects it to. Here, it was the latter.
> ** SIDEBAR **
> SIDEBAR: Don't be confused that Intel also refers to the Invalid Opcode exception (0x06) as a #UD or "undefined"
> By default, PCjs marks opcodes as invalid *only* if they have been confirmed invalid. PCjs considers the vast
majority of unused/undocumented opcodes to be merely "undefined" until I've seen them in the wild. An instruction will
be marked invalid if I either discover that real hardware treats it as an Invalid Opcode (by throwing the exception)
*or* that real software expects it to. For opcode 0x0F,0xFF, it was the latter.
> Don't be confused that Intel also refers to the Invalid Opcode exception (0x06) as the #UD ("Undefined")
opcode exception. Every opcode that triggers that exception is, by definition, defined: it's defined as invalid.
I consider it a misnomer to refer to any invalid instruction as "undefined".
@ -75,7 +77,12 @@ With all of the above changes in place, PCjs v1.18.4 is able to run Windows 95 S
completion. If you want to give it a spin yourself, start the machine below (click the "Run" button) and once it has
finished booting, run SETUP from drive B, where the first Windows 95 diskette is already loaded.
NOTE: This is a pre-release version of Windows 95, as I don't currently have the RTM version on diskette.
If, when it crashes (and it will), you're interested in examining the instructions that were executed prior to the
crash, use the "m int on" Debugger command prior to starting the machine. That will trigger allocation of the instruction
history buffer, which you can then dump at any time using the Debugger's "dh" command.
NOTE: The diskette images contain a pre-release version of Windows 95, as I don't currently have the RTM version on
diskette.
To be continued....