Assembly on Institute of Ineptitude

Assemble an MS-DOS .COM binary with DEBUG.EXE

Tue, 11 Nov 2025 14:57:43 -0700

The 8-Bit Guy has a video where he demonstrates the assembler built right into the monitor ROM on a C64.

This fascinated me and made me wonder… If I was stuck on an old MS-DOS machine and wanted to assemble a binary… Could I do it? Byte by byte? With what tool? Those of us stuck with “IBM compatible” PCs didn’t have no fancy monitor ROMs.

Turns out MS-DOS’ DEBUG.EXE is and was entirely up for the job.

ARM code for IPv4 Subnetting

Mon, 30 Dec 2024 00:00:00 +0000

ARM ASM code to find the network of an IP

ASCII case bit twiddling

Tue, 10 Dec 2024 21:49:28 -0700

If you pay a special sort of attention, you may notice that the upper-case and lower-case alphabet characters on the ASCII table are always exactly offset from each other by 32.

For example, upper-case A is 65, while the lowercase a is 97

As such the way the binary place value works out, you can manipulate the case of a character by setting or clearing the sixth bit… (2^5 = 32, we start counting bits and everything at 0, we’re programmers!)

Decimal 32 expressed as hexadecimal is 0x20 and expressed as binary is 00100000

Guess the number...

Fri, 05 Apr 2024 00:00:00 +0000

A quick demonstration of how one can use a debugger to interactively inspect variables in a running program…

Patching Binaries

Sat, 13 Mar 2021 11:19:00 -0600

Exercise Summary

The idea of this self-inflicted exercise was to see if I could inspect and patch a binary to change the outcome of some conditional code. In days of long past up I had found tutorials on how to patch a binary to bypass copyright protection schemes. The type that would ask a random question, where the answer found in a manual.

You know, the one you would have had if the game wasn’t recieved on bootleg floppy diskettes.