IBM AT 5170 Technical Reference page 440

• 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 Segment (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 (hex 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.
A program that requires timing information should use either the
time-of-day clock or the timing channels of the timer/counter.
The input frequency to the timer will be maintained at 1.19 MHz,
providing a constant time reference. Program loops should be
avoided.
Programs that use copy protection schemes should use the ROM
r--..
BIOS diskette calls to read and verify the diskette and should not
be timer dependent. Any method can be used to create the
diskette, although manufacturing capability should be considered.
9-14 Compatibility