INT 15 - SYSTEM - READ SYSTEM STATUS (CONVERTIBLE)
        AH = 43h
Return: AL = status bits (see below)

Bitfields for Convertible system status:
 bit 0  LCD detached
 bit 1  reserved
 bit 2  RS232/parallel adapter powered on
 bit 3  internal modem powered on
 bit 4  power activated by alarm
 bit 5  standby power lost
 bit 6  external power in use
 bit 7  power low

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