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Added proper clocking of VT100 keyboard UART, allowing us to restore the proper CPU speed (2.7Mhz) and maintain the correct cursor blink rate

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This commit is contained in:
Jeff Parsons 2016-08-22 09:14:34 -07:00
commit bca19fd8ed
39 changed files with 9641 additions and 35 deletions
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34
modules/pc8080/lib/chipset.js
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@ -172,25 +172,10 @@ ChipSet8080.SI1978 = {
* One of the many chips in the VT100 is an 8224, which operates at 24.8832MHz. That frequency is divided by 9
* to yield a 361.69ns clock period for the 8080 CPU, which means (in theory) that the CPU is running at 2.76Mhz.
*
* Hence the CPU component in the VT100's machine.xml SHOULD be defined as:
* Hence the CPU component in the VT100's machine.xml should be defined as:
*
* <cpu id="cpu8080" model="8080" cycles="2764800"/>
*
* where 2764800 = 24883200 / 9. Unfortunately, the VT100 ROM decrements a countdown value in memory to determine
* cursor blink rate, and if we use 2764800 cycles per second, the cursor blinks MUCH too fast. It's surprising that
* the VT100 doesn't rely on vertical retrace interrupts for blink rate. Perhaps the designers were concerned about
* consistency across 60Hz and 50Hz display modes, although that seems like a minor concern, considering that the
* alternative means the ROM is now tied to a specific CPU operating frequency. However, short of rewriting portions
* of the ROM, we have to deal with it.
*
* And we deal with it by lowering cycles per second to 1000000 (1Mhz). I'm guessing that in a real VT100, the 8080
* gets bogged down by other factors (eg, the Video Processor's DMA requests), but we don't simulate the hardware to
* that level of detail, so the easiest solution is to lower the effective clock speed.
*
* NOTE: If you've noticed that the VT100 cursor blinks unevenly, you're right, and it's by design: the ROM uses a
* countdown value for the cursor's "on" state that is twice as large as that for the cursor's "off" state, so it's
* "on" twice as long as it's "off".
*
* WARNING: The choice of clock speed has an effect on other simulated VT100 circuits; see the DC011 Timing Chip
* discussion below, along with the getVT100LBA() function.
*
@ -259,12 +244,12 @@ ChipSet8080.VT100 = {
*
* On p. 4-56, the DC011 Block Diagram shows 8 outputs labeled LBA0 through LBA7. From p. 4-61:
*
* Several of the LBAs are used as general purpose clocks in the VT100. LBA 3 and LBA 4 are used to generate
* Several of the LBAs are used as general purpose clocks in the VT100. LBA3 and LBA4 are used to generate
* timing for the keyboard. These signals satisfy the keyboard's requirement of two square-waves, one twice the
* frequency of the other, even though every 16th transition is delayed (the second stage of the horizontal
* counter divides by 17, not 16). LBA 7 is used by the nonvolatile RAM.
* counter divides by 17, not 16). LBA7 is used by the nonvolatile RAM.
*
* And on p. 4-62, timings are provided for the LBA0 through LBA7 when the VT100 is in 80-column mode; in particular:
* And on p. 4-62, timings are provided for the LBA0 through LBA7; in particular:
*
* LBA6: 16.82353us (when LBA6 is low, for a period of 33.64706us)
* LBA7: 31.77778us (when LBA7 is high, for a period of 63.55556us)
@ -958,10 +943,11 @@ ChipSet8080.prototype.doNVRCommand = function()
*/
ChipSet8080.prototype.inVT100Flags = function(port, addrFrom)
{
var b = this.bFlags;
/*
* The NVR_CLK bit is driven by LBA7 (ie, bit 7 from Line Buffer Address generation); see the DC011 discussion above.
*/
var b = this.bFlags;
b &= ~ChipSet8080.VT100.FLAGS.NVR_CLK;
if (this.getVT100LBA(7)) {
b |= ChipSet8080.VT100.FLAGS.NVR_CLK;
@ -969,18 +955,22 @@ ChipSet8080.prototype.inVT100Flags = function(port, addrFrom)
this.doNVRCommand();
}
}
b &= ~ChipSet8080.VT100.FLAGS.NVR_DATA;
if (this.bNVROut) {
b |= ChipSet8080.VT100.FLAGS.NVR_DATA;
}
b &= ~ChipSet8080.VT100.FLAGS.KBD_XMIT;
if (this.kbd && this.kbd.isTransmitterReady()) {
if (this.kbd && this.kbd.isVT100TransmitterReady()) {
b |= ChipSet8080.VT100.FLAGS.KBD_XMIT;
}
b &= ~ChipSet8080.VT100.FLAGS.UART_XMIT;
if (this.serial && this.serial.isTransmitterReady()) {
b |= ChipSet8080.VT100.FLAGS.UART_XMIT;
}
this.bFlags = b;
this.printMessageIO(port, null, addrFrom, "FLAGS", b);
return b;
@ -1045,7 +1035,7 @@ ChipSet8080.prototype.outVT100DC012 = function(port, b, addrFrom)
this.bDC012Blink = ~this.bDC012Blink;
break;
case 0x1:
// TODO: Clear vertical frequency interrupt
// TODO: Clear vertical frequency interrupt?
break;
case 0x2:
case 0x3: