$BA/186:   Current File - First Address (Device number)

Kernal-Reference:

 LDA $BA     : $F219 $F25B $F29D $F372 $F3E1 $F4AB $F4CB $F5ED $F60B $F646
               $FE07
 STA $BA     : $F327
 STX $BA     : $FE02

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