[RESOLVED] NASM - Trying to write a TSR->

[Peter Swinkels]

I decided to write a TSR for MS-DOS (specifically DOSBox) Why? Because I am curious as to what all that software writes to memory. DOSBox has a debugger version but I don't like it. Anyway:

; Memory Dumper TSR v1.00 - by: Peter Swinkels, ***2021***
; This terminate and stay resident program dumps all conventional memory to a file upon request.
ORG 0x100

Main:
MOV AH, 0x35          ; Retrieves the keyboard interrupt vector.
MOV AL, 0x16          ;
INT 0x21              ;

MOV AX, ES            ; Redirects the retrieved keyboard interrupt vector.
MOV DS, AX            ;
MOV DX, BX            ;
MOV AH, 0x25          ;
MOV AL, 0xFF          ;
INT 0x21              ;

MOV AX, CS            ; Sets the original keyboard vector to this program's memory dump function.
MOV DS, AX            ;
MOV DX, MemoryDumper  ;
MOV AH, 0x25          ;
MOV AL, 0x16          ;
INT 0x21              ;

MOV AH, 0x31          ; Terminates and stays resident.
INT 0x21              ;



MemoryDumper:
PUSHA                 ; Saves all registers.

MOV AH, 0x01          ; Skips the memory dump unless the F12 key has been pressed.
INT 0xFF              ;
CMP AX, 0x0086        ;
JNE Done              ;

MOV AX, CS
SUB AX, 0x0100
MOV DS, AX

MOV AH, 0x3C          ; Creates the output file.
MOV CX, 0x00          ;
MOV DX, OutputFile    ;
INT 0x21              ;
JC Done               ;

MOV AH, 0x3D          ; Opens the output file for writing.
MOV AL, 0x01          ;
MOV DX, OutputFile    ;
INT 0x21              ;
JC Done               ;

MOV BX, AX            ; Retrieves the filehandle.

MOV AX, DS            ; Saves the current data segment.
MOV ES, AX            ;

MOV WORD [MemorySegment], 0x0000    ; Sets the first memory block.

Dump:
   ES                          ; Sets the memory block to be written to the output file.
   MOV AX, [MemorySegment]     ;
   MOV DS, AX                  ;

   MOV AH, 0x40                ; Writes the memory block to the output file.
   MOV CX, 0xFFFF              ;
   MOV DX, 0x0000              ;
   INT 0x21                    ;
   JC Done                     ;

   ES                          ; Checks whether the last memory block has been reached.
   MOV AX, [MemorySegment]     ;
   CMP AX, 0xF000              ;
   JAE DumpFinished            ;

   ADD AX, 0x1000              ; Moves to the next memory block.
   ES                          ;
   MOV [MemorySegment], AX     ;
JMP Dump

DumpFinished:
MOV AH, 0x3E          ; Closes the output file.
INT 21h               ;
JC Done               ;

Done:
POPA                  ; Restores all registers.
INT 0xFF              ; Calls the redirected keyboard interrupt.
IRET                  ; Returns.

OutputFile DB "MemDump.dat", 0x00
MemorySegment DW 0x0000

It appears to set up the hook fine and I know the part that writes the memory to a file should work because I tested it separately. Does anyone see any obvious flaw? It could be any small thing I overlooked. No file is ever created.

EDIT:
I am using the Netwide Assembler (NASM)