INT 21 P - Phar Lap 386/DOS-Extender v3.0+ - READ/WRITE SYSTEM REGISTERS
        AX = 2535h
        EBX = direction (00h read registers, 01h write)
        DS:EDX -> system register record (see below)
Return: CF clear
Note:   this call is only available under MS Windows if PHARLAP.386 VDD is
          installed

Format of system register record:
Offset  Size    Description
 00h    DWORD   CR0
 04h  4 DWORDs  DR0,DR1,DR2,DR3
 14h  2 DWORDs  reserved
 1Ch  2 DWORDs  DR6,DR7
 24h  3 DWORDs  reserved

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