AH = 51h
Return: BX = Current PSP Segment

struct psp {
        char psp_int20[2];      /* 00h: exit */
        unsigned psp_msize;     /* 02h: memory size in paragraphs */
        char psp_res0[1];       /* 04h: XXX (0) */
        char psp_dos[5];        /* 05h: far call to dos */
        int (*psp_term)();      /* 0ah: terminate address */
        unsigned psp_tseg;      /* 0ch: terminate segment */
        int (*psp_break)();     /* 0eh: break address */
        unsigned psp_bseg;      /* 10h: break segment */
        int (*psp_error)();     /* 12h: error address */
        unsigned psp_eseg;      /* 14h: error segment */
        unsigned psp_ppsp;      /* 16h: parent psp segment */
        char psp_ofile[20];     /* 18h: open files, 0xff = unused */
        unsigned psp_envp;      /* 2ch: environment segment */
        char psp_res2[4];       /* 2eh: XXX */
        int psp_nfiles;         /* 32h: max open files */
        char *psp_aofile;       /* 34h: ofile address */
        unsigned psp_aoseg;     /* 36h: ofile segment */
        char psp_res3[24];      /* 38h: XXX */
        char psp_int21[3];      /* 50h: int 21, far return */
        char psp_res4[2];       /* 53h: XXX */
        char psp_xfcb1[7];      /* 55h: FCB #1 extension */
        char psp_fcb1[9];       /* 5ch: FCB #1 */
        char psp_xfcb2[7];      /* 65h: FCB #2 extension */
        char psp_fcb2[20];      /* 6ch: FCB #2 */
        char psp_dma[128];      /* 80h: Command Tail */
};
Note: under DOS 2.xx, this function cannot be invoked inside an INT 28h
handler
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