Atari 400 Technical Reference Manual page 102

3. The
as
tests to see if a diagnostic cartridge is in the A slot:
o Cartridge address BFFC
=
OO?
o The memory at BFFC is not RAM?
o B i t 7 of the byte at BFFD
=
1?
If all of the above tests are true. then control is passed to
the diagnostic cartridge via the vector at BFFE. No return is
expected.
4. The
as
determines the lowest memory address containing
non-RAM. by testing the first byte of every
4K
"black" to see
if the content can be complemented. If it can be complemented.
then the original value is restored and testing continues. If
it can't be complemented; then the content is assumed to be
the first non-RAM address in the system. The MSa of the
address is stored temporarily in TRAMSZ
[0006].
5. Zero is stored to all of the hardware register addresses shown
below (mast of that aren't decoded by the hardware):
DOOO through DOFF
D200 through D2FF
D300 through D3FF
D400 through D4FF
6.
The OS clears RAM from location 0008, to the address
determined in step
41
above.
7. The default value for the "noncartridge" control vectoT'
DOSVEC [OOOA] is set to point to the blackboard routine. At
the end of initialization. control is passed through this
vector if a cartridge does not take control.
8.
The coldstart flag COLDST [0244] is set to -1 (local use).
9.
The screen margins are set: left margin
=
2, right margin =
39, for a 38 character physical line. The maximum line size of
40 characters can be obtained by setting the margins to 0 and
39. The
as
insets the left margin because the two leftmost
columns of the video picture on many television sets are not
entirely visible on the screen.
10. The interrupt RAM vectors VDSLST [0200] through VVBLKD [0224]
are initialized. See Section
6
for the initialization values.
11.
Portions of the
as
RAM are set to their re~uired nonzero values
as shown below:
OPERATING SYSTEM C016555 -- Section 7
117