Loader for MSDOS EXE files, without MSDOS

ExeLDR.asm

;Made by FeDOS (FeDOS SubSystems)
;All Rights Reserved
;FeDOS (ICQ 313-997-813)
[BITS 16]
[ORG 0x0000]
__start:
;   mov cx, 0xB800
 ;  mov es, cx
;   mov bx, 0h
;   mov cx, cs
;   sub cx, 100h
;   mov ds, cx
;   mov ax, 00002h
;   int 10h
 ;  mov cx, 0xFA0
__loop:
;   mov al, [ds:bx]
;   mov [es:bx], al
;   inc bx
;   loop __loop
;   xor ah, ah
 ;  int 16h
    mov ax, cs
    mov bx, __start
    shr bx, 4
    add bx, 20h
    add ax, bx
    mov ds, ax
    mov ax, [ds:0]
    mov ebx, 0
__MZ:
          ;Length of file (last page)
    add ebx, 4
          ;Count of relocations
    inc ebx
    inc ebx
    mov ax, [ds:ebx]
    mov edx, __RELOCNT
    mov [cs:edx], ax
          ;HDR Length   
    inc ebx
    inc ebx
    mov eax, 0
    mov ax, [ds:ebx]
    shl eax, 4 
    mov edx, __HDRSIZE
    mov [cs:edx], eax
          ;MinMem, MaxMem
    add ebx, 4
          ;Get SS Segment address
    inc ebx
    inc ebx
    mov ax, [ds:ebx]
    mov edx, __SS
    mov [cs:edx], ax
          ;Get SP Start Value   
    inc ebx
    inc ebx
    mov ax, [ds:ebx]
    mov edx, __SP
    mov [cs:edx], ax
          ;ChkSum
    inc ebx
    inc ebx
          ;Get IP Start Value
    inc ebx
    inc ebx
    mov ax, [ds:ebx]
    mov edx, __IP
    mov [cs:edx], ax
          ;Get CS Segment address   
    inc ebx
    inc ebx
    mov ax, [ds:ebx]
    mov edx, __CS
    mov [cs:edx], ax
          ;Get First Offset
    inc ebx
    inc ebx
    mov ax, [ds:ebx]
    mov edx, __OFF
    mov [cs:edx], ax
          ;Get Overlay Number
    inc ebx
    inc ebx
          ;Looking for relocations
          ;Getting a start address
    mov edx, __RELOCNT
    mov ax, [cs:edx]
    cmp ax, 0
    je __RUN_

    mov ebx, __OFF
    mov bx, [cs:ebx]
          ;Cicle
__RELO:
          ;EAX <- Address of relocation
    mov eax, 0
    mov ax, [ds:ebx]
    add ebx, 2
    mov edx, 0
    mov dx, [ds:ebx]
    add ebx, 2
    shl edx, 4
    add eax, edx            ;EAX = Address of relocation
    
    xchg ebx, esi
          ;Changing Segment Relocation Address to real address
          ;Now - getting real address in memory -> ds:ebx
    mov ebx, 0
    mov edx, __HDRSIZE
    mov bx, [cs:edx]
    add ebx, eax
    mov dx, [ds:bx]

    xchg bx , di

    mov ecx, __CS
    mov cx, [cs:ecx]
    add dx, cx
    mov ecx, __HDRSIZE
    mov cx, [cs:ecx]
    shr cx, 4
    add dx, cx
    mov cx, ds
    add dx, cx

    xchg bx, di

    mov [ds:ebx], dx
    
    xchg ebx, esi
    jmp __CHECK
__RUN_:
    jmp __RUN
__CHECK:    
          ;Checking __RELOCNT>0 -> jump to RUN
    mov edx, __RELOCNT
    mov ax, [cs:edx]
    dec ax
    mov [cs:edx], ax
    cmp ax, 0
    je __RUN

    jmp __RELO    
__RUN:
          ;Running program
    mov bx, ds
    mov edx, __CS
    mov ax, [cs:edx]
    mov edx, __HDRSIZE
    xchg si, bx
    mov bx, [cs:edx]
    mov dx, bx
    xchg si, bx
    shr dx, 4
    add ax, dx
    add ax, bx
    mov edx, __IP
    mov dx, [cs:edx]
    mov ebx, off
    mov [cs:ebx], dx
    mov ebx, segmnt
    mov [cs:ebx], ax
    mov ds, ax
    mov es, ax
;   mov bx, 0f04h
;   mov cx, 0460h
;   mov dx, 0f04h
;   mov si, 0264h
;   mov di, 083b0h
;   mov bp, 0000h
;   mov sp, 03ffeh
    db 0EAh                ;jmp KernelSegB:[0000h]
    off dw 0000h           ;‘¬ҐйҐ­ЁҐ
    segmnt   dw 0000h      ;‘ҐЈ¬Ґ­в
__varlist:
__RELOCNT dw 0
__HDRSIZE dw 0, 0
__SS    dw 0
__SP    dw 0
__IP    dw 0
__CS    dw 0
__OFF   dw 0