Tag: electronics

This is a video showing some very long source code of a DOS program I wrote using Assembly Language.

This program is technically a 16 bit DOS .com program and yet it can access 32 bit addresses of the file it operates on because the LSEEK DOS call uses CX:DX as a 32 bit address by using two 16 bit registers. By modifying several parts of the program, I improved upon it greatly. It should theoretically be able to operate on any file less than 2 gigabytes even though the program itself never accesses more than 16 bytes at one time.

I may have just invented the world’s smallest and fastest DOS hex dumper and editor. The official gitlab repository has the source code seen in this video as well as the Linux 32 bit Assembly and the original C version I wrote first when I invented this program.

https://gitlab.com/chastitywhiterose/chastehex.git

I will have to make more videos showing examples of how it can be used, but I have a readme file in the repository that explains what it does and why I made it.

——–D-2142——————————-
INT 21 – DOS 2+ – “LSEEK” – SET CURRENT FILE POSITION
AH = 42h
AL = origin of move
00h start of file
01h current file position
02h end of file
BX = file handle
CX:DX = (signed) offset from origin of new file position
Return: CF clear if successful
DX:AX = new file position in bytes from start of file
CF set on error
AX = error code (01h,06h) (see #01680 at AH=59h/BX=0000h)
Notes: for origins 01h and 02h, the pointer may be positioned before the
start of the file; no error is returned in that case (except under
Windows NT), but subsequent attempts at I/O will produce errors
if the new position is beyond the current end of file, the file will
be extended by the next write (see AH=40h); for FAT32 drives, the
file must have been opened with AX=6C00h with the “extended size”
flag in order to expand the file beyond 2GB
BUG: using this method to grow a file from zero bytes to a very large size
can corrupt the FAT in some versions of DOS; the file should first
be grown from zero to one byte and then to the desired large size
SeeAlso: AH=24h,INT 2F/AX=1228h