Added new IBM PC AT configuration (8Mhz model, currently not booting)

blog/2014/10/13/README.md

@@ -0,0 +1,28 @@
+The 8Mhz IBM PC AT 5170 
+---
+I just added my first [IBM PC AT "Rev 3"](/configs/pc/machines/5170/ega/1152kb/rev3/) machine configuration
+to the list of [IBM PC Machine Configurations](/configs/pc/machines/), and not surprisingly, the new machine
+fails to boot.
+
+I call it "Rev 3" because it uses the 3rd [ROM BIOS](/devices/pc/bios/5170/) that IBM released for the PC AT,
+a revision that is supposed to include support for 3.5-inch 1.44Mb diskettes -- which would be great, because I
+have a number of 1.44Mb diskette images it would be nice to be able to read in a PCjs machine.
+
+Machines with this BIOS also ran at 8Mhz, so I've bumped the CPU speed up to 8,000,000 cycles/second.  It'll be
+interesting to see whether this BIOS also increased any of its hard-coded timing delay-loops as a result.
+
+Anyway, when I enabled ChipSet I/O port messages in the Debugger:
+
+	m chipset on
+	m port on
+
+I can see that the BIOS Power-On Self Test (POST) progresses nicely until it starts generating lots of port 0x61
+activity:
+
+	chipset.inPort(0x0061,8042_RWREG): 0x30 at F000:05A8
+	chipset.inPort(0x0061,8042_RWREG): 0x20 at F000:05AE
+
+I know what I'm going to be doing this afternoon now.
+
+*[@jeffpar](http://twitter.com/jeffpar)*  
+*October 13, 2014*