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INT 21 - Phar Lap 386/DOS-Extender - LOAD PROGRAM FOR DEBUGGING
        AX = 2512h
        DS:EDX -> pointer to ASCIIZ program name
        ES:EBX -> pointer to parameter block (see below)
        ECX = size in bytes of LDT buffer
Return: CF clear if successful
            EAX = number of segment descriptors in LDT
        CF set on error
            EAX = error code
                02h file not found or path invalid
                05h access denied
                08h insufficient memory
                0Ah environment invalid
                0Bh invalid file format
                80h LDT too small

Format of parameter block:
Offset  Size    Description
Input:
 00h    DWORD   32-bit offset of environment string
 04h    WORD    segment of environment string
 06h    DWORD   32-bit offset of command-tail string
 0Ah    WORD    segment of command-tail string
 0Ch    DWORD   32-bit offset of LDT buffer (size in ECX)
 10h    WORD    segment of LDT buffer
Output:
 12h    WORD    real-mode paragraph address of PSP (see also AH=26h)
 14h    WORD    real/protected mode flag
                0000h  real mode
                0001h  protected mode
 16h    DWORD   initial EIP value
 1Ah    WORD    initial CS value
 1Ch    DWORD   initial ESP value
 20h    WORD    initial SS value
 22h    WORD    initial DS value
 24h    WORD    initial ES value
 26h    WORD    initial FS value
 28h    WORD    initial GS value