54017 D301 PORTB


Since the XL and XE series no longer have a PORT B (on the
400/800 this controls joystick ports 3 and 4), this register is used
for LED control (1200XL only) and memory management.

You can disable the ROM between 49152-53247 ($C000-$CFFF)
and 55296-65535 ($D800-$FFFF) by setting bit 0 to 0 (the ROM
area becomes RAM; note that the area between $D000 and
$D7FF remains intact). However, unless another OS has been
provided, the system will crash at the next interrupt (1/60 sec-
ond later!), so you need to disable the interrupts first.

Bit 1 controls BASIC; if 0, BASIC is enabled, if 1, it is disabled
and the 8K RAM space enabled for program use. If you disable
BASIC within a BASIC program, you lock up. Disable BASIC dur-
ing a boot operation by holding down the OPTION key.

Bits 2 and 3 control the 1200XL LEDs; 0 means on, 1 means off.
LED 1 means the keyboard is disabled; LED 2 means the inter-
national character set is selected. In the 130XE, these bits are
used for bank switching 16K blocks of RAM. The 130XE allows
you to use the extra memory as video memory or program/
data memory. See the section on memory management in the
13OXE at the end of this chapter.

Bits 4-6 are reserved (unused) in the XL and 65XE. Bits 4 and 5
in the 13OXE are used to enable bank switching (see below).

Bit 7 controls the RAM region 20480-22527 ($5000-$57FF), nor-
mally enabled 1). When disabled 0), the OS ROM in that area is
enabled and access provided to the self-test code moved from
53248-55295 ($D000-$D7FF).