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:
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)
Code:
; 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 0x0000EDIT:
I am using the Netwide Assembler (NASM)