waters%viking.DEC@decwrl.ARPA@sri-unix.UUCP (08/24/84)
From: waters%viking.DEC@decwrl.ARPA (Lester Waters)
Below is a hastily compiled list of what I believe to be the salient
differences between the PC/XT and the PC/AT. I still have not completely
checked on some of the new ROM functionality, especially in the area of
multi-tasking support. If there are any significant errors, I will
send around an update.
0) 80286 vs. 8088/86. There are some real differences here that I will
cover later if requested.
1) Max memory of 15Mb (less 128Kb ROM space). IBM says that the AT only
supports 3MB. This is due to the fact that they only have 512Kb option
cards and 3Mb fills 6-slots. Watch for 3rd party 1-2Mb options.
2) 16 levels of interrupt support. Accomplished by adding a 2nd 8259.
IRQ9 is redirected to IRQ2 to compensate for the fact that IRQ2 is
no longer on the bus, but IRQ9 is. (I don't know why!)
3) 4 8-bit DMA channels (chl 0 is no longer used for refresh).
3 16-bit (word boundary only) DMA channels (2nd 8237 cascaded)
4) Real Time Clock/Calendar/Alarm/NVM (Motorola 146818) Large Lithium
battery attached to case with Velcro and motherboard with connector
5) Fixed Disk (not even closely compatible except at the ROM BIOS interface
and even there they have changed it slightly):
No DMA! 512 byte sector buffer that is directly accessible memory.
ST412 Interface, 5Mb transfer rate, WD1010 controller
Max support: 16 heads, 1024 cyl.
3:1 interleave instead of 7:1
6) 14.31818 Mhz clock independent of processor clock. Used for driving
8253 Timer and Video sub-system.
7) High density 1.2Mb diskettes (made by YE Data). 360 RPM, 15 Sectors,
96 tpi, 2-sided, can read 48tpi by double stepping. Supports status
change line.
8) 84 key keyboard (extra SYS REQ key). Identical software interface, but
electrically incompatible. Bi-directional + LED support in ROM.
9) Keylock freezes keyboard and cover. You can still power it off.
10) 64Kb ROM:
- Move arbitrary 32Kb of memory to/from addresses above 1Mb in
Real mode.
- Enter Protected mode (cannot use BIOS any longer).
- Multi-tasking hooks. (more later)
- TOD support with alarm function
- Video write string function added
- Status line change from high density diskette
- Read/Set drive type for diskettes and fixed disks.
- New ROM BIOS fixed disk support.
11) Minor changes to the 8-bit bus. Hardware depending on a specific
system clock frequency will no longer work. The 14Mhz clock is
still provided but is not synchronous with the system clock.