Initial commit (a clone of the jsmachines project as of v1.15.3)
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blog/2014/09/02/README.md
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blog/2014/09/02/README.md
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Minor Fixes and Additions
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---
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The following fixes were made in PCjs v1.15.1:
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1. Using the "User-defined URL" option when loading a disk image from a 3rd-party server was broken if the URL
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contained certain special characters; that should be fixed now, but be aware that only web servers (ie, URLs
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using the HTTP protocol) are supported. URLs that trigger a redirect may also not work (more testing required).
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2. Any errors that occur during the call to either *embedPC()* or *embedC1P()* should be properly displayed on the
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caller's page now.
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3. Two embedding helper functions have been added to provide more control over the machine startup
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and shutdown process:
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+ *enableEvents(boolean)*: pass *false* to disable delivery of all page events to all machines on the page,
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or *true* to re-enable;
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+ *sendEvent(string)*: pass *"init"*, *"show"* or *"exit"* to simulate the corresponding browser event
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(*onload*, *onpageshow* or *onbeforeunload*, respectively).
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If a page calls *enableEvents(false)* before calling *embedPC()*, all machine layouts will be instantiated
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but the machines themselves will not be initialized. When the page is ready, call *enableEvents(true)* to restore
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normal event processing, and if the browser has already sent the *onload* event, then call *sendEvent("init")*
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to manually initialize the machine(s).
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These two new functions are designed to assist in testing the starting up, shutting down and restarting of machines,
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by allowing scripts to control the overall process, without requiring use of the browser's back/forward/close controls.
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*[@jeffpar](http://twitter.com/jeffpar)*
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*September 2, 2014*
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blog/2014/09/13/README.md
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blog/2014/09/13/README.md
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The IBM PC AT: Alive and Booting
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---
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My first IBM PC AT (Model 5170) [Test Configuration](/configs/pc/machines/5170/ega/640kb/debugger/) finally
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boots to a PC-DOS prompt. The configuration uses the original [IBM Model 5170 ROM BIOS](/devices/pc/bios/5170/),
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dated January 10, 1984.
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Getting through the BIOS "POST" (Power-On Self Test) diagnostics was like running an obstacle course, with various
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tests derailing the simulation at every turn.
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Sometimes the problems were as simple as missing hardware. For example, I knew that the PC AT contained two DMA
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controllers, for a total of 8 DMA channels, but what I didn't know (or had forgotten) is that it also contained 16
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DMA page registers, some of which the BIOS uses as scratch registers. Since DMA page registers are accessed with
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I/O instructions that work identically in both real-mode and protected-mode, they obviously offer some advantages
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over RAM, especially when the BIOS hasn't yet tested all the RAM, or determined how much RAM is installed, or set up
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descriptors that allow the RAM to be accessed from protected-mode.
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Aside from the additional DMA Controller, other major new motherboard components on the PC AT included a second
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8259 Interrupt Controller, an 8042 Keyboard ("Kitchen Sink") Controller, and an MC146818 Real-Time Clock/CMOS chip.
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The Keyboard Controller and Real-Time Clock/CMOS components required the most tinkering to pass through the ROM BIOS
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gauntlet.
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For example, at one point, the BIOS ("[TEST.21](../../../../pubs/pc/reference/ibm/static/5170/techref/pages/IBM-5170-TECHREF 202.pdf)")
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reset the keyboard ("[KBD_RESET](../../../../pubs/pc/reference/ibm/static/5170/techref/pages/IBM-5170-TECHREF 212.pdf)"),
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which unmasked the keyboard IRQ and waited for an interrupt, using a loop where CX was initialized to zero and then
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decremented until either CX wrapped around to zero again *or* an interrupt occurred. The "TEST.21" code then assumed
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that if "KBD_RESET" returned zero in CX, no interrupt had occurred.
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Unfortunately, my Keyboard Controller was a bit too fast: it generated an interrupt as soon as the keyboard IRQ was
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unmasked; as a result, CX was never decremented, leaving it at zero.
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---
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Most of the effort getting to this point involved adding support for 80286 protected-mode. That work is still far
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from complete, but getting through multiple real-mode/protected-mode round trips in the BIOS was an important
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milestone. Some of the work was outside the CPU component, such as A20 support and processor reset via the 8042
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controller. Work inside the CPU component included:
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- New 80286 general-purpose instructions (eg, ENTER, LEAVE, new PUSH SP behavior, etc)
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- New protected-mode instructions (eg, ARPL, LGDT, LIDT, LAR, LSL, VERR, VERW, etc)
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- Protected-mode segment loading, addressing, and fault handling
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Another "feature" I spent considerable time on was ensuring that 80286 protected-mode support did not adversely
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real-mode performance, so that the PC and PC XT simulations still run (almost) as fast as before. PCjs dynamically
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reconfigures itself according to the requirements of the processor and platform it's emulating.
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---
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There's still no support for [LOADALL](/pubs/pc/reference/intel/80286/loadall/) or triple-fault resets, nor for
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call gates or task gates, nor for conforming code segments or expand-down data segments. 80286-specific cycle
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counts haven't been incorporated yet, either. The list of remaining 80286 features is long.
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And there's plenty of hardware support left to do: I haven't looked at the AT hard disk controller yet (which
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I believe is significantly different from the XT hard disk controller), and the keyboard *barely* works; the 8042
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Keyboard Controller and AT keyboard had a number of features that older PC/XT keyboards did not (like LEDs and
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programmable repeat rate).
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And there are plenty of issues to investigate. For example, PC-DOS is picking up the correct RTC time, but not the
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date (PCjs initializes the RTC to the browser's current date/time, unless a hard-coded date/time is specified in the
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machine XML). And diskette I/O seems a bit slow; I'm concerned that the BIOS is spinning its wheels somewhere
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unnecessarily. And even though the test machine is configured with 640Kb of RAM, the BIOS is reporting only 64Kb.
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Hmmmm.
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*[@jeffpar](http://twitter.com/jeffpar)*
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*September 13, 2014*
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