PC Extra Super CD 1998 January/PCPLUS131.iso/DJGPP/V2/DJLSR201.ZIP/src/stub/stub.asm

Assembly Source File | 1996-10-05 | 23.7 KB | 952 lines

; Copyright (C) 1996 DJ Delorie, see COPYING.DJ for details
; Copyright (C) 1995 DJ Delorie, see COPYING.DJ for details
; -*- asm -*-
;
; KLUDGE-WARNING!
;
; So you say you want to change this file, right?  Are you really sure
; that's a good idea?  Let me tell you a bit about the pitfalls here:
;
; * Some code runs in protected mode, some in real-mode, some in both.
; * Some code must run on a 8088 without crashing it.
; * Registers and flags may be expected to survive for a long time.
; * The code is optimized for size, not for speed or readability.
; * Some comments are parsed by other programs.
;
; You still want to change it?  Oh well, go ahead, but don't come
; crying back saying you weren't warned.
;
;-----------------------------------------------------------------------------
;  djgpp extender-less stub loader
;
;  (C) Copyright 1993-1995 DJ Delorie
;
;  Redistribution and use in source and binary forms are permitted
;  provided that: (1) source distributions retain this entire copyright
;  notice and comment, (2) distributions including binaries display
;  the following acknowledgement:  ``This product includes software
;  developed by DJ Delorie and contributors to the djgpp project''
;  in the documentation or other materials provided with the distribution
;  and in all advertising materials mentioning features or use of this
;  software, and (3) binary distributions include information sufficient
;  for the binary user to obtain the sources for the binary and utilities
;  required to built and use it. Neither the name of DJ Delorie nor the
;  names of djgpp's contributors may be used to endorse or promote
;  products derived from this software without specific prior written
;  permission.
;
;  THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
;  IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
;  WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
;
;  Revision history:
;
;  93/12/05 DJ Delorie    Initial version v2.00, requires DPMI 0.9
;  94/10/13 CW Sandmann v2.01, accumlated changes: 60K load bug, limits, cwsdpmi, optimization
;  94/10/29 CW Sandmann v2.03, M Welinder changes; cwsdpmi load anywhere, size decrease
;
   .copyright "The STUB.EXE stub loader is Copyright (C) 1993-1995 DJ Delorie. "
   .copyright "Permission granted to use for any purpose provided this copyright "
   .copyright "remains present and unmodified. "
   .copyright "This only applies to the stub, and not neccessarily the whole program.\n"
   .id
;
;-----------------------------------------------------------------------------
;  Interface to 32-bit executable:
;
;    cs:eip    according to COFF header
;    ds        32-bit data segment for COFF program
;    fs        selector for our data segment (fs:0 is stubinfo)
;    ss:sp    our stack (ss to be freed)
;    <others>    All unspecified registers have unspecified values in them.
;-----------------------------------------------------------------------------
;  This is the stubinfo structure.  The presence of this structure
;  indicates that the executable is a djgpp v2.00 executable.
;  Fields will never be deleted from this structure, only obsoleted.
;
    .org    0            ; just in case
stubinfo:
stubinfo_magic:                ; char [16]
    .db    "go32stub, v 2.00"    ; version may change, [0..7] won't
stubinfo_size:                ; unsigned long
    .dd    stubinfo_end        ; bytes in structure
stubinfo_minstack:            ; unsigned long
    .dd    0x40000            ; minimum amount of DPMI stack space (256K)
stubinfo_memory_handle:            ; unsigned long
    .dd    0            ; DPMI memory handle
stubinfo_initial_size:            ; unsigned long
    .dd    0            ; size of initial segment
stubinfo_minkeep:            ; unsigned short
    .dw    16384            ; amount of automatic real-mode buffer
stubinfo_ds_selector:            ; unsigned short
    .dw    0            ; our DS selector (used for transfer buffer)
stubinfo_ds_segment:            ; unsigned short
    .dw    0            ; our DS segment (used for simulated calls)
stubinfo_psp_selector:            ; unsigned short
    .dw    0            ; PSP selector
stubinfo_cs_selector:            ; unsigned short
    .dw    0            ; to be freed
stubinfo_env_size:            ; unsigned short
    .dw    0            ; number of bytes of environment
stubinfo_basename:            ; char [8]
    .db    8 .dup 0        ; base name of executable to load (asciiz if < 8)
stubinfo_argv0:                ; char [16]
    .db    16 .dup 0        ; used ONLY by the application (asciiz if < 16)
stubinfo_dpmi_server:            ; char [16]
    .db    "CWSDPMI.EXE\0\0\0\0\0"    ; used by stub to load DPMI server if no DPMI already present

    .align    4
stubinfo_end:

;-----------------------------------------------------------------------------
;  First, set up our memory and stack environment

    .start                ; execution begins here
    push    cs
    pop    ds
    mov    [stubinfo_ds_segment], ds

    mov    [psp_segment], es    ; save the PSP segment
    cld

;-----------------------------------------------------------------------------
;  Check that we have DOS 3.00 or later.  (We need this because earlier
;  versions don't supply argv[0] to us and will scrog registers on dpmi exec).
    mov    ah, 0x30
    int    0x21
    cmp    al, 3
    jae    dos3ok
    mov    dx, msg_bad_dos
    jmpl    error
dos3ok:
    mov    [dos_major], al

;-----------------------------------------------------------------------------
;  Resize memory in case we need to exec a DPMI server

resize_again:
    mov    bx, end_of_memory    ; does not include PSP
    mov    ax, [stubinfo_minkeep]
    cmp    bx, ax            ; is our program big enough to hold it?
    jae    @f1
    mov    bx, ax
@f1:
    mov    [stubinfo_minkeep], bx    ; store for reference
    inc    bh            ; add 256 bytes for PSP
    mov    cx, 0xff04        ; 0xff is for below
    shr    bx, cl            ; bytes to paragraphs

    mov    ah, 0x4a        ; ES = PSP segment from above
    int    0x21            ; resize our memory block
    jnc    @f1            ; did it work?
    shl    bx,cl            ; calculate smaller [keep] value
    dec    bh
    mov    [stubinfo_minkeep], bx
    jmp    resize_again        ; and try again
@f1:

;-----------------------------------------------------------------------------
;  Scan environment for "PATH=" and the stub's full name after environment

    mov    es, es:[0x2c]        ; get environment segment
    xor    di, di            ; begin search for NUL/NUL (di = 0)
;    mov    cx, 0xff04        ; effectively `infinite' loop
    xor     al, al
    .db    0xa9            ; "test ax,...." -- skip 2 bytes
scan_environment:
    repne
    scasb                ; search for NUL
    cmpw    es:[di], 0x4150        ; "PA"
    jne    not_path
    scasw
    cmpw    es:[di], 0x4854        ; "TH"
    jne    not_path
    scasw
    cmpb    es:[di], '='
    jne    not_path
    inc    di            ; Point to PATH contents
    mov    [path_off], di        ; save for later
    dec    di            ; in case the PATH is empty
not_path:
    scasb
    jne    scan_environment    ; no, still environment
    scasw                ; adjust pointer to point to prog name

;-----------------------------------------------------------------------------
;  Get DPMI information before doing anything 386-specific

    push    es
    push    di
    xor    cx, cx            ; flag for load attempt set cx = 0
    jz    @f2            ; We always jump, shorter than jmp
@b1:
    mov    dx, msg_no_dpmi
    jmpl    error
@b2:
    or    cx, cx
    jnz    @b1            ; we already tried load once before
    inc    cx
    call    load_dpmi
    jc    @b1
@f2:
    mov    ax, 0x1687        ; get DPMI entry point
    int    0x2f
    or    ax, ax
    jnz    @b2            ; if 0 then it's there
    and    bl, 1            ; 32 bit capable?
    jz    @b2
@f3:
    mov    [modesw], di        ; store the DPMI entry point
    mov    [modesw+2], es
    mov    [modesw_mem], si
    pop    di
    pop    es

;-----------------------------------------------------------------------------
;  Now, find the name of the program file we are supposed to load.

;    xor    ah, ah            ; termination character (set above!)
    call    store_env_string    ; copy it to loadname, set bx

    mov    [stubinfo_env_size], di
    mov    [loadname_nul], si    ; remember nul so we can change it to $
    cmpb    stubinfo_basename[0], 0
    je    no_symlink

;-----------------------------------------------------------------------------
;  Replace the stub's file name with the link's name after the directory

    mov    cx, 8            ; max length of basename
    mov    di, stubinfo_basename    ; pointer to new basename
@b1:
    mov    al, [di]        ; get next character
    inc    di
    or    al, al            ; end of basename?
    je    @f1
    mov    [bx], al        ; store character
    inc    bx
    loop    @b1            ; eight characters?
@f1:
    movd    [bx+0], 0x4558452e    ; append ".EXE"
    add    bx, 4
    mov    [bx], al        ; al = 0
    mov    [loadname_nul], bx    ; remember nul so we can change it to $

no_symlink:

;-----------------------------------------------------------------------------
;  Load the COFF information from the file

    mov    ax, 0x3d00        ; open file for reading
    mov    dx, loadname
    int    0x21
    jcl    error_no_progfile    ; do rest of error message

@f1:
    mov    [program_file], ax    ; store for future reference

    mov    bx, ax
    mov    cx, exe_header_length
    mov    dx, exe_header
    mov    ah, 0x3f        ; read EXE header
    int    0x21

    xor    dx, dx            ; dx = 0
    xor    cx, cx            ; offset of COFF header

    mov    ax, [exe_magic]
    cmp    ax, 0x014c        ; COFF?
    je    file_is_just_coff
    cmp    ax, 0x5a4d        ; EXE magic value
    jnel    error_not_exe

    mov    dx, [exe_sectors]
    shl    dx, 9            ; 512 bytes per sector
    mov    bx, [exe_bytes_last_page]
    or    bx, bx            ; is bx = 0 ?
    je    @f1
    sub    dh, 2            ; dx -= 512
    add    dx, bx
@f1:

file_is_just_coff:            ; cx:dx is offset
    mov    coff_offset[0], dx
    mov    coff_offset[2], cx
    mov    ax, 0x4200        ; seek from beginning
    mov    bx, [program_file]
    int    0x21

    mov    cx, coff_header_length
    mov    dx, coff_header
    mov    ah, 0x3f        ; read file (bx = handle)
    int    0x21

    cmp    ax, coff_header_length
    jne    @f2
    cmpw    coff_header[coff_magic], 0x014c
@f2:
    jnel    error_not_coff

    mov    eax, aout_header[aout_entry]
    mov    [start_eip], eax

    mov    ecx, [coff_offset]

    mov    eax, text_section[s_scnptr]
    add    eax, ecx
    mov    [text_foffset], eax

    mov    eax, data_section[s_scnptr]
    add    eax, ecx
    mov    [data_foffset], eax

    mov    ebx, bss_section[s_vaddr]
    mov    eax, bss_section[s_size]
    add    ebx, eax
    mov    eax, 0x00010001
    cmp    ebx, eax
    jae    @f1
    mov    ebx, eax         ; ensure 32-bit segment
@f1:
    add    ebx, 0x0000ffff        ; ensure 64K rounded
    xor    bx, bx            ; clear rounded bits
    mov    [stubinfo_initial_size], ebx

;-----------------------------------------------------------------------------
;  Set up for the DPMI environment

    call    include_umb
    mov    bx, [modesw_mem]
    or    bx, bx
    jz    no_dos_alloc

    mov    ah, 0x48        ; allocate memory for the DPMI host
    int    0x21
    jcl    error_no_dos_memory_umb
    mov    es, ax

no_dos_alloc:
    call    restore_umb
    mov    ax, 1            ; indicates a 32-bit client
    callf    [modesw]        ; enter protected mode

    jcl    error_in_modesw

;-----------------------------------------------------------------------------
; We're in protected mode at this point.

    mov    [stubinfo_psp_selector], es
    mov    [stubinfo_cs_selector], cs
    mov    ax, ds
    mov    [stubinfo_ds_selector], ax
    mov    es, ax

    xor    ax, ax            ; AX = 0x0000
    mov    cx, 1
    int    0x31            ; allocate LDT descriptor
    jc    @f2
    mov    [client_cs], ax

    xor    ax, ax            ; AX = 0x0000
;    mov    cx, 1            ; already set above
    int    0x31            ; allocate LDT descriptor
@f2:
    jcl    perror_no_selectors
    mov    [client_ds], ax

; Try getting a DPMI 1.0 memory block first, then try DPMI 0.9
; Note:  This causes the Borland Windows VxD to puke, commented for now with ;*
;*    mov    ax, 0x0504
;*    xor    ebx, ebx        ; don't specify linear address
    mov    ecx, stubinfo_initial_size[0]
;*    mov    edx, 1            ; allocate committed pages
;*    int    0x31            ; allocate memory block
;*    jc    try_old_dpmi_alloc
;*    mov    client_memory[0], ebx
;*    mov    stubinfo_memory_handle[0], esi
;*    jmp    got_dpmi_memory
try_old_dpmi_alloc:
    mov    ax, 0x0501
    mov    bx, stubinfo_initial_size[2]
;    mov    cx, stubinfo_initial_size[0]    ;Set above
    int    0x31            ; allocate memory block
    jcl    perror_no_dpmi_memory
    mov    client_memory[2], bx
    mov    client_memory[0], cx
    mov    stubinfo_memory_handle[2], si
    mov    stubinfo_memory_handle[0], di
got_dpmi_memory:

    mov    ax, 0x0007
    mov    bx, [client_cs]        ; initialize client CS
    mov    cx, client_memory[2]
    mov    dx, client_memory[0]
    int    0x31            ; set segment base address

    mov    ax, 0x0009
;    mov    bx, [client_cs]        ; already set above
    mov    cx, cs            ; get CPL
    and    cx, 0x0003
    shl    cx, 5
    push    cx            ; save shifted CPL for below
    or    cx, 0xc09b        ; 32-bit, big, code, non-conforming, readable
    int    0x31            ; set descriptor access rights

    mov    ax, 0x0008
;    mov    bx, [client_cs]        ; already set above
    mov    cx, stubinfo_initial_size[2]
    dec    cx
    mov    dx, 0xffff
    int    0x31            ; set segment limit

    mov    ax, 0x0007
    mov    bx, [client_ds]        ; initialize client DS
    mov    cx, client_memory[2]
    mov    dx, client_memory[0]
    int    0x31            ; set segment base address

    mov    ax, 0x0009
;    mov    bx, [client_ds]        ; already set above
    pop    cx            ; shifted CPL from above
    or    cx, 0xc093        ; 32-bit, big, data, r/w, expand-up
    int    0x31            ; set descriptor access rights

    mov    ax, 0x0008
;    mov    bx, [client_ds]        ; already set above
    mov    cx, stubinfo_initial_size[2]
    dec    cx
    mov    dx, 0xffff
    int    0x31            ; set segment limit

;-----------------------------------------------------------------------------
;  Load the program data

    mov    ax, 0x0100
    mov    bx, 0x0f00        ; 60K DOS block size
    int    0x31            ; allocate DOS memory
    jnc    @f1
    cmp    ax, 0x0008
    jnel    perror_no_dos_memory
    mov    ax, 0x0100        ; try again with new value in bx
    int    0x31            ; allocate DOS memory
    jcl    perror_no_dos_memory
@f1:
    mov    [dos_block_seg], ax
    mov    [dos_block_sel], dx
    shl    bx, 4            ; paragraphs to bytes
    mov    [dos_block_size], bx

    mov    esi, [text_foffset]    ; load text section
    mov    edi, text_section[s_vaddr]
    mov    ecx, text_section[s_size]
    call    read_section

    mov    esi, [data_foffset]    ; load data section
    mov    edi, data_section[s_vaddr]
    mov    ecx, data_section[s_size]
    call    read_section

    mov    es, [client_ds]        ; clear the BSS section
    mov    edi, bss_section[s_vaddr]
    mov    ecx, bss_section[s_size]
    xor    eax,eax
    shr    ecx,2
    .addrsize
    rep
    stosd

    mov    ah,0x3e
    mov    bx, [program_file]
    int    0x21            ; close the file

    mov    ax, 0x0101
    mov    dx, [dos_block_sel]
    int    0x31            ; free up the DOS memory

    push    ds
    pop    fs
    mov    ds, [client_ds]
    .opsize
    jmpf    fs:[start_eip]        ; start program

;-----------------------------------------------------------------------------
;  Read a section from the program file

read_section:
    mov    eax, esi        ; sector alignment by default
    and    eax, 0x1ff
    add    ecx, eax
    sub    si, ax            ; sector align disk offset (can't carry)
    sub    edi, eax        ; memory maybe not aligned!

    mov    [read_size], ecx    ; store for later reference
    mov    [read_soffset], edi

    call    zero_regs
    mov    dpmi_regs[dr_dx], si    ; store file offset
    shr    esi, 16
    mov    dpmi_regs[dr_cx], si
    mov    bx, [program_file]
    mov    dpmi_regs[dr_bx], bx
    movw    dpmi_regs[dr_ax], 0x4200
    call    pm_dos            ; seek to start of data

; Note, handle set above
    mov    ax, [dos_block_seg]
    mov    dpmi_regs[dr_ds], ax
    movw    dpmi_regs[dr_dx], 0    ; store file offset
read_loop:
    movb    dpmi_regs[dr_ah], 0x3f
    mov    ax, read_size[2]    ; see how many bytes to read
    or    ax, ax
    jnz    read_too_big
    mov    ax, read_size[0]
    cmp    ax, [dos_block_size]
    jna    read_size_in_ax        ; jna shorter than jmp
read_too_big:
    mov    ax, [dos_block_size]
read_size_in_ax:
    mov    dpmi_regs[dr_cx], ax
    call    pm_dos            ; read the next chunk of file data

    xor    ecx, ecx
    mov    cx, dpmi_regs[dr_ax]    ; get byte count
    mov    edi, [read_soffset]    ; adjust pointers
    add    [read_soffset], ecx
    sub    [read_size], ecx

    xor    esi, esi        ; esi=0 offset for copy data
    shr    cx, 2            ; ecx < 64K
    push    ds
    push    es
    mov    es, [client_ds]
    mov    ds, [dos_block_sel]
    .addrsize
    rep
    movsd
    pop    es
    pop    ds

    add    ecx, [read_size]    ; ecx zero from the rep movsd
    jnz    read_loop

    ret

;-----------------------------------------------------------------------------
;  Routine to check al for delimiter

test_delim:
    cmp    al, ':'            ; watch for file name part
    je    @f3
    cmp    al, '/'
    je    @f3
    cmp    al, '\\'
@f3:
    ret

;-----------------------------------------------------------------------------
;  Copy string from environment to loadname.
;   On entry: di = environment offset
;             ah = termination character (null also does)
;   On exit:  bx = pointer to one character after last observed file delimiter
;             di = pointer to one character after last copied
;             si = pointer to the copied termination character
;             al = terminating character

store_env_string:
    mov    si, loadname        ; pointer to buffer
    mov    bx, si            ; in case no delimiters
@b1:
    mov    al, es:[di]        ; copy a character to buffer
    inc    di
    mov    [si], al
    cmp    al, ah            ; end of file name?
    je    @f1
    or    al, al            ; end of file name?
    je    @f1
    inc    si
    call    test_delim
    jne    @b1
    mov    bx, si            ; remember pointer to first char of
    je    @b1            ; next name component (shorter than jmp)
@f1:
    ret

;-----------------------------------------------------------------------------
;  Most errors come here, early ones jump direct (8088 instructions)

error_no_progfile:
    mov    dx, msg_no_progfile
    jmp    error_fn

error_not_exe:
    mov    dx, msg_not_exe
    jmp    error_fn

error_not_coff:
    mov    dx, msg_not_coff
;    jmp    error_fn

error_fn:
    push    dx
    mov    bx, [loadname_nul]    ; error, print file name
    movb    [bx], '$'
    mov    bx, loadname
    jmp    @f1

error_no_dos_memory_umb:
    call    restore_umb
error_no_dos_memory:
    mov    dx, msg_no_dos_memory
    jmp    error

error_in_modesw:
    mov    dx, msg_error_in_modesw
    jmp    error

perror_no_selectors:
    mov    dx, msg_no_selectors
    jmp    error

perror_no_dpmi_memory:
    mov    dx, msg_no_dpmi_memory
    jmp    error

perror_no_dos_memory:
    mov    dx, msg_no_dos_memory
;    jmp    error

error:
    push    dx
    mov    bx, err_string
@f1:
    call    printstr
    pop    bx
    call    printstr
exit:
    mov    bx, crlfdollar
    call    printstr
    mov    ax, 0x4cff        ; error exit
    int    0x21

printstr1:
    inc    bx
    mov    ah, 2
    int    0x21
printstr:
    mov    dl, [bx]
    cmp    dl, '$'
    jne    printstr1
    ret

crlfdollar:
    .db    13,10,'$'
;-----------------------------------------------------------------------------
;  DPMI utility functions

zero_regs:
    push    ax
    push    cx
    push    di
    xor    ax, ax
    mov    di, dpmi_regs
    mov    cx, 0x19
    rep
    stosw
    pop    di
    pop    cx
    pop    ax
    ret

pm_dos:
    mov    ax, 0x0300        ; simulate interrupt
    mov    bx, 0x0021        ; int 21, no flags
    xor     cx, cx            ; cx = 0x0000 (copy no args)
    mov    edi, dpmi_regs
    int    0x31
    ret

;-----------------------------------------------------------------------------
;  load DPMI server if not present
;   First check directory from which stub is loaded, then path, then default
;   On entry di points to image name

path_off:
    .dw    0            ; If stays zero, no path

load_dpmi:
    xor    ah, ah            ; Copy until this character (=0)
    call    store_env_string    ; copy stub image to "loadname"
    mov    si, bx            ; remove name so we can add DPMI name
    mov    di, [path_off]        ; Pointer to path contents (next try)
    jmp    @f2
loadloop:
    mov    ah, ';'            ; Copy until this character
    call    store_env_string    ; to "loadname"
    cmp    si, loadname        ; anything there?
    je    do_exec            ; final try (no path) let it return
    mov    al, [si-1]
    call    test_delim        ; is final character a path delimiter
    je    @f2
    movb    [si], '\\'        ; no, add separator between path & name
    inc    si
@f2:
    call    do_exec            ; copy our name to string and try load
    jc    loadloop
    ret

;-----------------------------------------------------------------------------
; add the string CWSDPMI to path ending

do_exec:
    call    include_umb
    mov    bx, stubinfo_dpmi_server
@b1:
    mov    al, [bx]
    mov    [si], al
    inc    bx
    inc    si
    or    al, al
    jne    @b1
;    movw    [si], 0x0a0d        ;debug
;    movb    [si+2], '$'        ;debug

    push    es            ; Save in case of failure
    push    di

;memory saving - use dpmi_regs as a temporary parameter block
    push    ds
    pop    es            ;zero_regs needs es set
    call    zero_regs
    mov    bx, dpmi_regs
    mov    [bx+4], ds        ;segment of command tail
    mov    [bx+2], bx        ;offset (point to zero)

    mov    dx, loadname
;    mov    ah, 9            ;debug
;    int    0x21            ;debug
    mov    ax, 0x4b00        ;Do program exec
    int    0x21
    pop    di
    pop    es
    jc    @f1            ;carry set if exec failed

    mov    ah, 0x4d        ;get return code
    int    0x21
    sub    ax, 0x300        ;ah=3 TSR, al=code (success)
    neg    ax            ;CY, if not originally 0x300
@f1:
    jmp    restore_umb        ;called func. return for us.

;-----------------------------------------------------------------------------
; Make upper memory allocatable.  Clobbers Ax and Bx.

include_umb:
    cmpb    [dos_major], 5        ; Won't work before dos 5
    jb    @f1
    mov    ax, 0x5800        ; get allocation strategy
    int    0x21
    mov    [old_strategy],al
    mov    ax, 0x5802        ; Get UMB status.
    int    0x21
    mov    [old_umb],al
    mov    ax, 0x5801
    mov    bx, 0x0080        ; first fit, first high then low
    int    0x21
    mov    ax, 0x5803
    mov    bx, 0x0001        ; include UMB in memory chain
    int    0x21
@f1:
    ret

; Restore upper memory status.  All registers and flags preserved.

restore_umb:
    pushf
    cmpb    [dos_major], 5        ; Won't work before dos 5
    jb    @f1
    push    ax
    push    bx
    mov    ax, 0x5803        ; restore UMB status.
    mov    bl,[old_umb]
    xor    bh, bh
    int    0x21
    mov    ax, 0x5801        ; restore allocation strategy
    mov    bl,[old_strategy]
    xor    bh, bh
    int    0x21
    pop    bx
    pop    ax
@f1:
    popf
    ret

;-----------------------------------------------------------------------------
;  Stored Data
err_string:
    .db    "Load error: $"
msg_no_progfile:
    .db    ": cannot open$"
msg_not_exe:
    .db    ": not EXE$"
msg_not_coff:
    .db    ": not COFF (Check for viruses)$"
msg_no_dpmi:
    .db    "no DPMI - Get csdpmi*b.zip$"
msg_no_dos_memory:
    .db    "no DOS memory$"
msg_bad_dos:
    .db    "need DOS 3$"
msg_error_in_modesw:
    .db    "can't switch mode$"
msg_no_selectors:
    .db    "no DPMI selectors$"
msg_no_dpmi_memory:
    .db    "no DPMI memory$"

;-----------------------------------------------------------------------------
;  Unstored Data, available during and after mode switch

last_generated_byte:

    .align    512            ; Align ourselves to a sector
                    ;  boundary for startup speed.
    .bss                ; data after this isn't in file.

modesw:                    ; address of DPMI mode switch
    .dd    0
modesw_mem:                ; amount of memory DPMI needs
    .dw    0

program_file:                ; file ID of program data
    .dw    0

text_foffset:                ; offset in file
    .dd    0

data_foffset:                ; offset in file
    .dd    0

start_eip:                ; EIP value to start at
    .dd    0
client_cs:                ; must follow start_eip
    .dw    0
client_ds:
    .dw    0

client_memory:
    .dd    0

dos_block_seg:
    .dw    0
dos_block_sel:
    .dw    0
dos_block_size:
    .dw    0

read_soffset:
    .dd    0
read_size:
    .dd    0

dpmi_regs:
    .db    0x32 .dup 0
dr_edi = 0x00
dr_di  = 0x00
dr_esi = 0x04
dr_si  = 0x04
dr_ebp = 0x08
dr_bp  = 0x08
dr_ebx = 0x10
dr_bx  = 0x10
dr_bl  = 0x10
dr_bh  = 0x11
dr_edx = 0x14
dr_dx  = 0x14
dr_dl  = 0x14
dr_dh  = 0x15
dr_ecx = 0x18
dr_cx  = 0x18
dr_cl  = 0x18
dr_ch  = 0x19
dr_eax = 0x1c
dr_ax  = 0x1c
dr_al  = 0x1c
dr_ah  = 0x1d
dr_efl = 0x20
dr_es  = 0x22
dr_ds  = 0x24
dr_fs  = 0x26
dr_gs  = 0x28
dr_ip  = 0x2a
dr_cs  = 0x2c
dr_sp  = 0x2e
dr_ss  = 0x30

;-----------------------------------------------------------------------------

    .align    16            ; so that stack ends on para boundary
    .dw    128 .dup 0
    .stack

;-----------------------------------------------------------------------------
; At one time real mode only data.  Header stuff now used during image load.

psp_segment:
    .dw    0

loadname_nul:                ; offset of NUL so it can become '$'
    .dw    0
loadname:                ; name of program file to load, if it
    .db    81 .dup 0        ; gets really long ok to overwrite next

exe_header:                ; loaded from front of loadfile
exe_magic:
    .dw    0
exe_bytes_last_page:
    .dw    0
exe_sectors:
    .dw    0
exe_header_length = . - exe_header

coff_offset:
    .dd    0            ; from start of file

coff_header:                ; loaded from after stub
    .db    20 .dup 0
aout_header:
    .db    28 .dup 0
text_section:
    .db    40 .dup 0
data_section:
    .db    40 .dup 0
bss_section:
    .db    40 .dup 0
coff_header_length = . - coff_header

old_strategy:
    .db    0
old_umb:
    .db    0

dos_major:
    .db    0

    .align    16            ; Align ourselves to a paragraph
end_of_memory:                ; resize is done early so must keep all

;-----------------------------------------------------------------------------
;  structure definitions
;

coff_magic    = 0            ; from coff header

aout_entry    = 16            ; from aout header

s_paddr        = 8            ; from section headers
s_vaddr        = 12
s_size        = 16
s_scnptr    = 20