INT 1A - PCMCIA Socket Services - REGISTER STATUS CHANGE CALLBACK
        AH = 81h
        DS:DX -> callback routine (see below) or 0000h:0000h to disable
Return: CF clear if successful
            AH destroyed
        CF set on error
            AH = error code (see AH=80h"PCMCIA")
Note:   the callback will be invoked on any socket changes whose notification
          has not been disabled with the status change enable mask; it may be
          invoked either while processing a hardware interrupt from the adapter
          or while processing the following Socket Services request
SeeAlso: AH=80h"PCMCIA",AH=82h"PCMCIA"

Callback routine invoked with:
        AL = adapter number
        BH = status change interrupt enable mask (see below)
        BL = socket number
        DH = current socket status (see below)
        DL = current card status (see below)
Return: all registers preserved
Notes:  the callback may be invoked during a hardware interrupt, and may not
          call on Socket Services
        the callback will be invoked once for each socket with a status change

Bitfields for status change enable mask:
 bits 0,1 reserved (0)
 bit 2  ejection request
 bit 3  insertion request
 bit 4  battery dead change
 bit 5  battery warning change
 bit 6  ready change
 bit 7  card detect change

Bitfields for current socket status:
 bit 0  reserved (0)
 bit 1  card locked
 bit 2  card ejection request pending
 bit 3  card insertion request pending
 bit 4  card ejection complete
 bit 5  card insertion complete
 bit 6  reserved (0)
 bit 7  card changed

Bitfields for current card status:
 bit 0  write protected
 bits 1-3 reserved (0)
 bit 4  battery voltage detect 1 (battery dead)
 bit 5  battery voltage detect 2 (battery warning)
 bit 6  ready
 bit 7  card detect

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