.model tiny
  .data
    ByteCode db 61440 DUP (?)
    Buffer db 0
    Filename db "TEST.COM", "$"
    hFile dw ?
  .code
  org 100h
  start:
    mov ah, 3Dh
    mov al, 0
    lea dx, Filename
    int 21h
    mov hFile, bx
    
    ReadCode:
      mov ah, 3Fh
      mov bx, hFile
      mov cx, 1
      lea dx, ByteCode
      int 21h
      
      cmp ax, 0h
      je GoOn
      jmp ReadCode
      
    GoOn:
      lea dx, ByteCode+256
      jmp dx

    mov ah, 4ch
    int 21h
  end start
    

;; COM file loader;; David Lindauer, March 25, 1997;;  This should run MOST com files, if not all of them.  The only;  especially tricky part is marking the newly allocated memory;  segments with an appropriate owner so that the memory will be;  released properly when the program exits.;;  Note: there are some aspects of DOS compatability I haven't taken;  care of here; for example the program name should show up right after;  the environment with a fully qualified path; and it should show up;  again right after the command line without the path.  I may have missed;  other stuff as well...;;        .model tiny        .386        .code        org 100hstart:        jmp go        db 0C4h         ; word aligning the stack;_____________________________________________________________________;; Internal stack;        dw      128 DUP ('?')tos:;_____________________________________________________________________;; Internal variables;query db        10,13,"Please enter name of file to execute: ",'$'response db     40,0,40 DUP (?);_____________________________________________________________________;; Failure comes here;errmsg db       10,13,"failed.",10,13,'$'failure:        push cs                 ; print a message if something fails        pop ds        mov dx,offset errmsg        mov ah,9        int 21h        mov ax,4c01h        int 21h;_____________________________________________________________________;; Prepare an empty environment.  COMMAND.COM copies the environment of; the parent to a new hunk of memory and throws some data in after it;; We just make an empty environment;prepare_env:        mov bx,1                ; need one paragraph        mov ah,48h        int 21h        jc failure        mov es,ax               ; Zero it out        sub di,di        sub ax,ax        mov cx,8        rep stosw        ret;_____________________________________________________________________;; Allocate all useable memory to our new prog; Actually not 100% compatible with what DOS does but close enough; for IBM PC computers.;allocate_allmem:        mov bx,-1               ; find out how much mem        mov ah,48h        int 21h        cmp bx,1000h            ; At least 64K?        jc failure        push bx                 ; Allocate it        mov ah,48h        int 21h         mov es,ax        pop bx                  ; returning size of block        jc failure        ret;_____________________________________________________________________;; Initialize the new PSP;; Note: this should handle most DOS .COM files.  Of course it; makes little sense to go to all this trouble if we aren't running DOS; programs .   BTW this should let them run, but it may keep them from; important information such as command lines and environments;prepare_psp:        push ax                 ; start by zeroing it out        mov cx,128        sub ax,ax        sub di,di        rep stosw        pop ax        mov word ptr es:[2ch],ax        ; Pointer to our environment        mov word ptr es:[0],20cdh       ; null return to DOS        mov ax,es        add ax,bx        mov word ptr es:[2],ax          ; Next free block (usually A000)        mov byte ptr es:[5],9ah         ; CALLF;; Now we grab stuff from the original PSP.  Usually DOS initializes; this stuff with values out of the appropriate vectors but I got lazy...;        mov si,6                        ; vectors 21h-24h copy from old PSP        mov di,6        mov cx,8        rep movsw        mov ax,ds:[16h];; This next field would typically be set to point at itself or; the parent which did the spawning; I set it to the parent of; this loader program;        mov word ptr es:[16h],ax        ; Common parent        mov byte ptr es:[18h],1         ; Handle table        mov byte ptr es:[19h],1        mov byte ptr es:[1ah],1        mov byte ptr es:[1bh],0        mov byte ptr es:[1ch],2        mov di, 1dh                     ; blank out unused entries (handles)        mov al,0ffh                                                         mov cx,15        rep stosb        mov word ptr es:[32h],20        ; 20 handles available        mov word ptr es:[34h],18h       ; Point at handle table        mov word ptr es:[36h],ES        ;        mov dword ptr es:[38h],-1       ; reserved        mov word ptr es:[50h],21CDh     ; dummy DOS call        mov byte ptr es:[52h],0CBH;; The rest of this stuff has to be modified if you put in command line support;        mov di,5dh                      ; Now blank out the FCB file names        mov al,20h        mov cx,8        rep stosb        mov di,6dh        mov cx,8        rep stosb        mov word ptr es:[80h],0D00h     ; Indicate no command line        ret;_____________________________________________________________________;; Reclassify the memory for the PSP and ENV as being owned by the PSP; If we don't do this the memory may not get freed up... which typically; results in a lockup.;rename_arena:        push es        push es        mov ax,es:[2ch]        dec ax        mov es,ax        pop ax        mov es:[1],ax        dec ax        mov es,ax        pop ax        mov es:[1],ax        mov es,ax        ret;_____________________________________________________________________;; Program entry point;go:        mov sp,offset tos               ; Switch to internal stack;; This is the standard ASM prelude for freeing up memory the program; isn't using;        mov bx,offset eop               ; Free up unused memory        add bx,15        shr bx,4        mov ah,4ah        int 21h;; Everything up to here is to adjust for the fact we are already a com file;;; First we make the environment.  We are making an empty one but it should; get copied either from the master environment or from the parent's ; environment        call prepare_env                ; Memory initialize;; Allocate program memory.  We are going to allocate anything we can; get our hands on.  We HAVE to allocate this rather than just default; to the pre-allocated memory because some programs depend on the PSP; being at the beginning of an arena block;        push es        call allocate_allmem        pop ax;; Now initialize all relevant fields of the PSP;        call prepare_psp;; Retag the arena entries for the memory we have allocated so it will; be freed properly;        call rename_arena               ; rename the arena entries;; Put a prompt and get file name;        mov dx,offset query             ; Get the file name        mov ah,9        int 21h        mov dx,offset response        mov ah,10        int 21h        mov bh,0        mov bl,[response+1]        mov [bx+response+2],0;; Read in the file;        mov ax,3d00h                    ; Open the file        mov dx,offset response+2        int 21h        mov bx,ax        jc failure        mov ax,3f00h                    ; Read the file        mov dx,100h        push es        pop ds        mov cx,0FF00h        int 21h        jc failure        mov cx,ax                       ; Szie of prog in CX        mov ax,3e00h                    ; close the file        int 21h        jc failure                push cx        push es;; Now we free the resources of this loader program.  Note this will; leave a memory gap- a better solution would to have been to free it; BEFORE allocating memory.  Of course then we have to move ourselves; out of the way;        mov es,cs:[2ch]                 ; free our environment        mov ah,49h        int 21h                push cs                         ; Free our program seg        pop es        mov ah,49h        int 21h;; Now we have to inform DOS that the active PSP has changed.  It is normal; to eventually restore the PSP in TSRs but we won't be coming back; later.;        mov bx,ds                       ; Inform DOS about the new PSP        mov ah,50h        int 21h        pop es        pop cx;; Now we set up the segment registers and such.  The registers we; are initializing are as follows:;; DS,ES,SS,Sp,CS,IP; BX:CX;; AFAIK that is all that has to be initialized.  But maybe you ought to; write a short program to dump the registers at startup of a com file; and see?  Don't depend on DEBUG... it zeros the registers but DOS; doesn't do that.        cli                             ; Set stack pointer to new process        push ds        pop ss        mov sp,0fff0h        sti        sub bx,bx                       ; Put the null in case they        push bx                         ; use RET to exit        push ds                         ; set the CS:IP pair to run prog        push 100h        retfeop:        end start