26 February 2022

Porting RLE8

I am going back in time (again) and playing with some of my old source code: In the days of MS-DOS, 16 bit operating systems and 640 kB of RAM, I created a whole bunch of MS-DOS utilities and tools, mostly written in Turbo Pascal. In the last 20 years I have only ported a few of them. From time to time I miss one of these old tools - but never miss it enough to invest the time to write it from scratch. Last year I came across p2c, a Pascal to C translator by Dave Gillespie. Oh such joy - and I used it to port my old tools. One tool I used - which I had not created myself - was RLE8 by Shaun Case, Public Domain 1991.

art Moderne (licensed CC BY-NC-ND by Nadine)RLE8
RLE stands for run-length encoding compression. It is a form of data compression in which repeated values are represented by a count and a single instance of the value. It is a very simple form of compression and in the nineties it sometimes reduced disk and memory space significantly. It was used in early versions of Windows BMP for bitmap compression named BI_RLE8: An RGB format that used run-length encoding (RLE) compression for bitmaps with 8 bits per pixel. The compression used a 2-byte format consisting of a count byte followed by a byte containing a colour index. There were versions for 4 and 8 bit image data, RLE4 and RLE8 respectively. "RLE compression was used in the stone-age" as one forum comment reads and today there not much interest in it. The only reference I found was for benchmarking highly optimised code.

Finding the Original Source
Finding the original source code was difficult. It is in the nature of the modern WWW that new pages appear and old ones disappear. Fortunately the RLE8.EXE printed the name of its author and its license: Shaun Case 1991 Public Domain. After some googling I found an article about it, which later turned out to be the contents of the Readme someone had reposted almost ten years later on GameDev. Eventually I found the Retro Computing Archive with its collection of CD-ROMs containing shareware and Public Domain software from the late 80's and 90's. RLE8_SC. Win! (The Retro Computing Archive is great, many of its ZIP files are unpacked and therefore crawled by Google which helped me find it.)

Porting from Turbo C
The code compiled without issues, but included a header I did not have, dir.h, a header from Borland's Turbo C. I guess this is the biggest issue when porting old code - calls to non-standard library functions. I missed fnsplit, a function which split file specifications into component parts. While I could have created the function myself - an excellent opportunity to practice my C - I searched more and luckily someone had created it already. Thank you Robert B. Stout. Robert granted license to use his source files to create royalty-free programs, which RLE8 is. After adding some more of Robert's code, and removing code which was not needed, RLE8 compiled and linked, even with my pedantic settings of gcc -std=c99 -pedantic -pedantic-errors -Wall -Wextra. I loved it. Witness the power of C, a 50 years old programming language, still moving forward.

Download original and modified sources together with binaries for DOS and Windows x86.

1 comment:

Unknown said...

Cool, what you are doing Peter, I think I also have still some Turbo Pascal code from my studies, which we can convert, when we will meet again... What was the day ?