AH = 4Bh
        AL = subfunction
            0 = load and execute program
            2 = load (Internal) but do not execute
            3 = load overlay; do not create PSP
        DS:DX = filename
        ES:BX = parameter block
               AL =
                0 => word segment environment pointer
                     dword command line pointer
                     dword FCB 1
                     dword FCB 2
                3 => word segment load address
                     word segment relocation factor
Return: CF = 1 if error
            AX = Error Code

struct exec {
        unsigned exec_magic;    /* 0x4d, 0x5a signature */
        unsigned exec_isr;      /* image size remainder (mod 512) */
        unsigned exec_size;     /* file size in pages (512) */
        unsigned exec_nrel;     /* number of relocation items */
        unsigned exec_hsize;    /* header size in paragraphs */
        unsigned exec_min;      /* minimum extra paragraphs */
        unsigned exec_max;      /* maximum extra paragraphs */
        unsigned exec_ss;       /* stack segment */
        unsigned exec_sp;       /* stack offset */
        unsigned exec_cksum;    /* word checksum of entire file */
        unsigned exec_pc;       /* initial pc */
        unsigned exec_cs;       /* code segment */
        unsigned exec_orel;     /* offset of relocation table */
        unsigned exec_ovno;     /* overlay number */
};
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