INT 21 - Phar Lap 386/DOS-Extender - EXECUTE PROGRAM
AX = 25C3h
ES:EBX -> pointer to parameter block (see below)
DS:EDX -> pointer to ASCIIZ program filename
Return: CF clear if successful
all registers unchanged
CF set on error
EAX = error code
01h function code in AL is invalid ???
02h file not found or path invalid
05h access denied
08h insufficient memory to load program
0Ah environment invalid
0Bh invalid file format

Format of parameter block:
Offset  Size    Description
00h    DWORD   32-bit offset of environment string
04h    WORD    segment selector of environment string
06h    DWORD   32-bit offset of command-tail string
0Ah    WORD    segment selector of command-tail string

2125C3 - Phar Lap 386/DOS-Extender - EXECUTE PROGRAM