Introduction
Reverse engineering the TI-84 Plus OS is a fascinating journey into the circuitry of this legendary calculator. Launched in 2004, the TI-84 Plus has become an indispensable tool for students in mathematics and sciences. But what lies beneath its simple interface? What we are about to discover is a sophisticated architecture that maximizes the use of its limited resources.
The TI-84 Plus Architecture
The TI-84 Plus is powered by a Zilog Z80 processor, a classic yet powerful choice for embedded devices. This processor features a 16-bit address bus, limiting the logical space to 64 KiB, while the calculator boasts 1 MiB of flash memory and 128 KiB of RAM. To bridge this gap, the TI-84 employs a four-slot paging scheme and a system-call mechanism called "bcall" allowing code on a 16 KiB flash page to call routines on any other page.
The Four Pillars
Everything the user encounters on the TI-84 Plus — from the homescreen to TI-BASIC programs — is built on four fundamental pillars:
- Paging and bcalls: These mechanisms enable access to code and data beyond 64 KiB, ensuring efficient memory management.
- The floating-point engine: Using 9-byte BCD registers, it handles all mathematical operations via OP1 to OP6 registers.
- The variable system (VAT): The Variable Allocation Table catalogs named objects such as reals, lists, matrices, strings, programs, etc.
- The tokenizer and parser: TI-BASIC is stored as 1- and 2-byte tokens, and the parser executes them.
Input/Output Systems
Surrounding these pillars, various input/output subsystems facilitate user interaction:
- IM1 Interrupts: Manage timing, automatic power-down, the cursor, and the ON key.
- LCD Driver: Manages LCD ports and screen buffers.
- Keypad Scanner: Facilitates user input.
- Link Port: Enables communication with other devices.
Challenges and Prospects
Reverse engineering this OS is not without challenges. Verifying hypotheses from TI-83+/84+ documentation and assembly analysis requires meticulous attention. However, it offers a deep understanding of how hardware limitations can be overcome through smart software engineering.
Conclusion
Exploring the TI-84 Plus OS teaches us the importance of optimization and ingenuity in software engineering. For tech developers and entrepreneurs, it offers valuable lessons on maximizing available resources.
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