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Format of memory control block:
Offset  Size    Description
 00h    BYTE    block type: 5Ah if last block in chain, otherwise 4Dh
 01h    WORD    PSP segment of owner, 0000h if free, 0008h if belongs to DOS
 03h    WORD    size of memory block in paragraphs
 05h  3 BYTEs   unused

---DOS 3.x---

 08h  8 BYTEs   unused

---DOS 4.x---

 08h  8 BYTEs   ASCII program name if PSP memory block, else garbage
                null-terminated if less than 8 characters

        Notes:  under DOS 3.1+, the first memory block is the DOS data
                segment, containing installable drivers, buffers, etc.
                under DOS 4.x it is divided into subsegments, each with its
                own memory control block (see below), the first of which
                is at offset 0000h