Specifications

Developer Technical Support May 1990
Apple IIGS
#69: The Ins and Outs of Slot Arbitration 3 of 7
Slot Number
F E D C B A 9 8 7 6 5 4 3 2 1 0
Slot
0 = Internal; 1 = External
Zero
Call Type Identifier
00 = Slot Request
11 = Select by Bit Encoded Slot Configurat
i
Zero
Slot Dependent or Slot Independent
Figure 1–Slot Number and Bit Definitions
Bit Encoded Slot Configurations
Every call to the Slot Arbiter returns (on exit) a miniature picture of the slot configuration in the
X register (as it was on entry). This picture has one bit set for each of the 14 slots; if the bit is
set, then the corresponding slot is switched in. Bits 0 and 8 are reserved and are always clear.
This picture is called a Bit Encoded Slot Configuration.
Since each external slot has the same number as an internal port (with bit 3 set), and since such
pairs share the same address space, it follows that both of them may not be enabled at the same
time. For example, port 5 and slot 5 ($D) both may not be enabled. This makes the high byte of
the Bit Encoded Slot Configuration the eXclusive-OR of the low byte (excluding bits 0 and 8,
which are always clear). Figure 2 illustrates the Bit Encoded Slot Configuration.