NASMusage

NASMusage is a Windows x86-64 learning repo for combining NASM assembly with C. The main project is a small TCP networking probe: C handles Winsock networking, while NASM routines process the received bytes with low-level buffer operations.

What This Demonstrates

• Windows x64 calling convention between C and NASM
• Building NASM object files and linking them with C
• Winsock TCP connection setup from C
• Passing network buffers into assembly routines
• Byte-level checksum, byte counting, and in-place ASCII transformation in NASM
• Cleaner project structure for small mixed-language systems programs

Project Layout

include/
  net_asm.h              C declarations for NASM routines

src/
  tcp_probe.c            Winsock TCP client demo
  net_asm.asm            Assembly helper routines used by the client

examples/
  hello.asm              Minimal printf example
  console_grid.asm       Windows console rendering demo

build.ps1                PowerShell build script
build.bat                Batch build script
LICENSE                  GPL-3.0 license

Requirements

• Windows
• NASM
• GCC/MinGW-w64 or another C compiler that can link with Winsock

Make sure both nasm and gcc are available on your PATH.

Build

Using PowerShell:

.\build.ps1

Using Command Prompt:

build.bat

Manual build command:

nasm -f win64 src\net_asm.asm -o build\net_asm.obj
gcc src\tcp_probe.c build\net_asm.obj -Iinclude -lws2_32 -o build\tcp_probe.exe

Run

Default target:

.\build\tcp_probe.exe

Custom host, port, and path:

.\build\tcp_probe.exe example.com 80 /

The program sends an HTTP HEAD request, receives the first response chunk, and then calls NASM routines to:

• calculate a 32-bit additive checksum
• count newline bytes
• uppercase the response preview in place

Assembly API

The C program calls these NASM functions:

uint32_t asm_checksum32(const uint8_t *buffer, size_t length);
size_t asm_count_byte(const uint8_t *buffer, size_t length, uint8_t value);
void asm_uppercase_ascii(char *buffer, size_t length);

These functions follow the Windows x64 ABI:

• RCX, RDX, R8, and R9 hold the first four integer or pointer arguments
• RAX holds the return value
• caller-saved registers may be overwritten

Extra Examples

Build the hello example:

nasm -f win64 examples\hello.asm -o build\hello.obj
gcc build\hello.obj -o build\hello.exe

Build the console grid example:

nasm -f win64 examples\console_grid.asm -o build\console_grid.obj
gcc build\console_grid.obj -o build\console_grid.exe -lkernel32 -lmsvcrt

Notes

This repo is intentionally small and educational. The networking code is kept in C because Winsock setup is clearer there; the assembly focuses on data-processing routines where register usage, pointer arithmetic, and ABI details are easier to study.

License

This repository is licensed under the GPL-3.0 license. See LICENSE for details.