For a while now I've been wanting to write a cycle-exact emulator for the original IBM PC and XT machines that are the direct ancestors of the machine I'm writing this on (and, almost certainly, the machine you're reading this on). I've written an 8088 emulator with cycle timings that I think are plausible but I have no idea if they're correct or not. The exact details of the timings don't seem to be published anywhere (at least not on the internet) so the only way to determine if they are correct is to compare against real hardware.<\/p>\n
So, when I saw a cheap XT for sale on eBay recently, I made a spur-of-the-moment bid, and won it. It is a bit beaten up - the case was bent in one place, the speaker cone was falling off and the end of one the ISA slots was broken off. All these problems were easy to fix with a hammer and a bit of superglue, though. It lacks keyboard and monitor, which makes it rather difficult to do anything useful with it, but it did come with all sort of weird and wonderful cards:<\/p>\n
It also comes with a floppy drive and a hard drive which makes the most horrendous metal-on-metal grinding sounds when it spins up and down (so I'll be very surprised if it still works).<\/p>\n
This machine has clearly been a real working PC for someone rather than a perfectly preserved museum piece - it tells stories. At some point the original MDA card failed and was replaced with a clone, the RAM was upgraded, it's been used as a terminal for a mainframe, and at some point the owner read about this device that improves your video output, bought one, found it didn't work with his MDA card but just left it in there.<\/p>\n
Due to lack of keyboard and monitor (neither my TV nor my VGA monitor can sync to MDA frequencies) I haven't got it to boot yet. I tried to use the scan doubler with the MDA and my VGA monitor (connecting the mono video signal via the red channel of the doubler) and the monitor was able to sync but the output was garbage - I guess the doubler is either broken or only works with CGA frequencies. If it does work with CGA then it'll be useful for seeing the TTL CGA output (though I'll have to put something together to convert the RGBI digital signals to analogue RGB - with proper fix up for colour 6 of course).<\/p>\n
I ordered a CGA card but decided to see if I could jerry-rig something up in the meantime. I programmed my Arduino to pretend to be an XT keyboard and also the \"manufacturing test device\" that IBM used in their factories to load code onto the machine during early stage POST (it works by returning 65H instead of AAH in response to a keyboard reset). I then used this to reprogram the CRTC of the MDA to CGA frequencies (113 characters of 9 pixels at 16MHz pixel clock for 18 rows (14 displayed) of 14-scanline characters plus an extra 10 scanlines for a total of 262 scanlines). The sources for this are on github<\/a>.<\/p>\n Next, I connected the hsync, vsync, video and intensity lines to a composite output stage like the one in the CGA card (it's just a transistor and a few resistors) and put some junk in video memory. Amazingly, it works - there is a barely legible picture. Even more amazingly, it is in colour! On the same breadboard I was doing this on, I had a Colpitts oscillator driving a 3.5795MHz (NTSC colour burst frequency) crystal from an earlier experiment (before my XT arrived I was trying to get colour TV output from my Arduino). This oscillator wasn't connected to anything, but the very fact that that frequency was bouncing around nearby was enough to turn off the colour killer circuit in the TV and allow it to interpret certain horizontal pixel patterns as colours.<\/p>\n The colours themselves aren't actually related to the picture in any useful way - the hsync and crystal aren't in sync so the phase (and hence the hues) drift over time. In fact, by applying slight heat and\/or pressure to the crystal with my fingers, I can change the frequency slightly and make the hue phase drift rate faster or slower with respect to the frame rate, and even stop it altogether (though it's still not a multiple of the line rate so the colours form diagonal patterns).<\/p>\n