Texas Microsystems, Inc. 2003 - BIOS Startup Routine

Machines Advanced Diagnostics program £or the AT can be
used.
3.1.1 BIOS Startup Routine
The BIOS is a collection o£ driver and initialization
so£tware stored in ROM on the CPU board. The BIOS is
designed to be totally compatible with so£tware written
£or the IBM personal computers. The IIO drivers are
invoked through the so£tware interrupt structure o£ the
80286 processor.
The largest portion o£ the BIOS so£tware program,
(approximately 4K bytes), is devoted to computer
startup. During this procedure, the memory and IIO
ports are tested and initialized. I£ everything goea
well and no errors are discovered, an operating system
is loaded and executed £rom the disk or other mass
storage device. The BIOS startup routine is as
£ollows:
1. The 80286 executes a Jump instruction to the BIOS
start point.
2. The CPU £lags are tested. The 80286 processor
register set is tested. I£ either o£ these tests
£ails, the CPU halts.
3. Video controllers are stabilized.
4. The £ollowing
occurs during
halts.
tests are per£ormed. I£ a £ailure
anyone o£ these tests, the CPU
5. The shutdown byte in CMOS RAM is tested £or
reliability.
6. The 8254 timer is tested.
7. DMA controllers are tested.
8. DMA page registers are tested.
9. DMA controllers are initialized.
10. Memory re£resh is tested.
the CPU halts.
I£ a £ailure occurs,
11. Bus A20 line is disabled.
Preliminary AT Users Manual
9 11/26/86