INT 15 - MultiDOS Plus - GET SYSTEM BLOCK
        AH = 15h
Return: BX:AX -> system block (see below)

Format of MultiDOS Plus 4.0 system block:
Offset  Size    Description
 00h    WORD    segment of system control block
 02h    WORD    redirection flag set by /NOREDIRECT
 04h    WORD    no-INT 10 flag set by /NO10
 06h    DWORD   old INT 10
 0Ah    DWORD   new INT 10
 0Eh    DWORD   pointer to WORD with current TCB offset (see AH=13h)
 12h    DWORD   pointer to WORD with idle task TCB offset
 16h    DWORD   pointer to WORD with foreground TCB offset
 1Ah    DWORD   pointer to WORD with MultiDOS TCB offset
 1Eh    WORD    Task Control Block size
 20h    WORD    number of TCBs
 22h    WORD    flag: EMS present
 24h    WORD    EMS page frame base segment
 26h    WORD    16K pages in EMS page frame
 28h    WORD    base segment for conventional memory tasks
 2Ah    WORD    conventional memory size in paragraphs
 2Ch    DWORD   pointer to list of queue pointers