[0F] - Increment secondary memory of the currently active window.

C1/042E: C2 31        REP #$31                  Flag manipulation.
C1/0430: 20 01 03     JSR $0301                 A = Root address of current active window entry.
C1/0433: 18           CLC 
C1/0434: 69 1F 00     ADC #$001F                Add 1F to the root (address of secondary memory).
C1/0437: AA           TAX                       Transfer to X...
C1/0438: BD 00 00     LDA $0000,X               Load twice? Weird.
C1/043B: BD 00 00     LDA $0000,X
C1/043E: 1A           INC                       Increment.
C1/043F: 9D 00 00     STA $0000,X               Store back.
C1/0442: 60           RTS                       End.