IBM 5170 Technical Reference page 388

• Control of the sound using port hex 61, and the sound channel
of the timer/counter. A program can control timer/counter
channels 0 and 2, ports hex 40,42, and 43. A program must
not change the value in port hex 41, because this port controls
the dynamic-memory refresh. Channel 0 provides the
time-of-day interrupt, and can also be used for timing short
intervals. Channel 2 of the timer/counter is the output for the
speaker and cassette ports. This channel may also be used for
timing short intervals, although it cannot interrupt at the end
of the period.
• Control of the Game Control Adapter, port hex 201
Note: Programs should use the timer for delay on the
paddle input rather than a program loop.
• Interrupt Mask Register (IMR), port hex 21, can be used to
selectively mask and unmask the hardware features.
The following information pertains to absolute memory locations.
• Interrupt Vectors (hex O)--A program may change these to
point at different processing routines. When an interrupt ~
vector is modified, the original value should be retained. If the
interrupt, either hardware or program, is not directed toward
this device handler, the request should be passed to the next
item in the list.
• Video Display Buffers (hex BOOOO and B8000)-- For each
mode of operation defined in the video display BIOS, the
memory map will remain the same. For example, the bit map
for the 320 x 200 medium-resolution graphics mode of the
Color/Graphics Monitor adapter will be retained on any
future adapter that supports that mode. If the bit map is
modified, a different mode number will be used.
• ROM BIOS Data Area (40:0)--Any variables in this area will
retain their current definition, whenever it is reasonable to do
so. IBM may use these data areas for other purposes when the
variable no longer has meaning in the system. In general,
ROM BIOS data variables should be read or modified through
BIOS calls whenever possible, and not with direct access to
the variable.
9-10 Compatibility