Ok, found the reason why it compiled with the leading zeros. In the main file dskpatch.asm I had the end directive but changing it to end disk_patch solved the issue. disk_patch being the name of the main procedure in dskpatch.asm. All references link correctly now as it loads into memory as it …

Thanks, that makes sense. Although I can't understand why it is being compiled that way. The problem must be with the tools not dosbox.

In file explorer properties it is called an ms-dos application (com) Here is a screen capture of hexedit and debug showing correct offset in hexedit but wrong in debug. The LEA is pointing to 053F but the data is actually at 063F I have been using hexedit to change all these points and it then works …

This is the entry point. There are 8 other modules linked. Probably a bit too much code to post.

CGROUP GROUP CODE_SEG, DATA_SEG ASSUME CS:CGROUP, DS:CGROUP CODE_SEG SEGMENT PUBLIC ORG 100h EXTRN CLEAR_SCREEN:NEAR, READ_SECTOR:NEAR EXTRN INIT_SEC_DISP:NEAR, WRITE_HEADER:NEAR EXTRN WRITE_PROMPT_LINE:NEAR, DISPATCHER:NEAR DISK_PATCH PROC NEAR CALL CLEAR_SCREEN CALL WRITE_HEADER CALL READ_SECTOR …

Hi, I am working through Peter Nortons Assembly language book for the IBM and there is a project that is built as you proceed called dskpatch which allows inspecting sectors of a disk and editing them. I have been using dosbox to test as I don't have original hardware that this book was written for( …