A project log for YATAC78 - The WWW TTL Computer

Retro computer built from 1978-era TTL logic chips. Internet capable with built in web browser and server

Alastair HewittAlastair Hewitt 06/17/2019 at 03:370 Comments

Focus shifted to building the ROM this weekend. Here's a quick overview of the contents:

  1. Native Program (128k) - code executed natively by machine.
  2. ALU (56k) - lookup tables containing ALU results.
  3. Relocatable Code (40k) - code executed by interpreter.
  4. Fonts (32k) - binary fonts used in text mode and dithering patterns used in hi-res graphics mode.

The machine uses an 8-bit program counter (PC) and 8-bit page register (Pg). An additional bit of state (bank) is held by the CPU state machine to define two banks of 64k, providing the 128k of address space.

The upper 128k contains various lookup tables. These can be split into two general sections: 96k of ALU functions and 32k of fonts. The ALU contains 56k of math and logic functions, including four 8-bit wide binary functions (ADD, SUB, AND, OR), three 4-bit wide binary functions (MUL, DIV, MOD), and 48 unary functions (discussed in earlier logs).

The remaining 40k of ALU functions are reserved for relocatable code. These act like the other functions but return a byte of code as the result of the function. This may sound odd, but it is the most efficient method of reading data from the ROM using the Harvard Architecture. The alternative is to write a program that would load a the byte of code as an operand, write it to the RAM, then increment a pointer to the next memory location. It would take at least 3 bytes of native code to write each operand to memory, requiring almost all 128k of program memory to load 40k of interpreted code.

The final part of the ROM are the fonts. There are two sets of four fonts. The first set use 8x8 glyphs and the second use 8x16 glyphs. The initial plan was to have bold and italic fonts, but this really isn't possible at 8-pixels wide! There are some other options though and these can be broken down as follows:

  1. Thick Serif
  2. Thin Serif
  3. Thick San-serif
  4. Thin San-serif

Two sets of these fonts were selected from The Ultimate Oldschool PC Font Pack. It was quite challenging to process the old bitmap files, but this excellent resource was able to pull out the data and even render it as simple text files. From there a script packs the fonts into the 32k font area of the ROM. A test script was used to verify the ROM and generate the following PNG:

All the fonts can be seen one after another. The first 4 fonts only take up 8 lines and the other 8 lines are used for dithering patterns in the hi-res graphics mode (discussed in the last log).