$E853/59475:   IRQ routine for serial bus

Jump from $FE73:

E853: AD 01 18  LDA $1801       ; read port A, erase IRQ flag
E856: A9 01     LDA #$01
E858: 85 7C     STA $7C         ; set flag for 'ATN received'
E85A: 60        RTS

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