$0244-$0249/580-585:   Pointer: Last char on channel.

   Points to the last character read/written in buffer.

ROM-Reference:

 CMP $0244,Y: $D144 $D3E3 $D3F5 $E12C $E147
 DEC $0244,X: $ED53
 LDA $0244,Y: $CD65 $D13A $D15D $ED71
 STA $0244,X: $DC62 $DCD6 $E197 $E1AE $ECB0 $ED50
 STA $0244,Y: $CBFF $CD4D $CD6D $D197 $E022

 STA $0249  : $CB37 $E6FC

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