Routines
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$13D5: Determine the address of a blackboard buffer
Used by the routines at $1421 and $1445.
Input
Y Blackboard identifier (0, 1, 2)
Output
($17,$18) Blackboard buffer address
$13D5 LDA #$44 Prepare to address the Reading Room blackboard buffer at $4420.
$13D7 STA $18
$13D9 LDA #$20
$13DB STA $17
$13DD TYA Transfer the blackboard identifier to A.
$13DE BEQ $13F2 Branch if it's the Reading Room blackboard (0).
$13E0 LDA $17 Prepare to address the White Room blackboard buffer at $4426.
$13E2 CLC
$13E3 ADC #$06
$13E5 STA $17
$13E7 CPY #$02 Do we want the Exam Room blackboard (2)?
$13E9 BNE $13F2 Branch if not.
$13EB LDA $17 Prepare to address the Exam Room blackboard buffer at $442C.
$13ED CLC
$13EE ADC #$06
$13F0 STA $17
$13F2 RTS
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