Efficient Implementation of Lookup Reference Tables in GCBASIC

Introduction

This section explores the efficient implementation of lookup reference tables in embedded systems, specificially GCBASIC, focusing on the use of PROGMEM memory to store fixed data sets. It addresses common misconceptions about data storage and initialization, compares different methods of data handling, and provides advanced techniques for optimizing memory usage.

Lookup reference tables are essential in embedded systems for storing fixed data sets that can be accessed during runtime. This section aims to clarify the correct implementation of these tables, debunking common myths and providing practical solutions for efficient data management.

Conventional Misconceptions

A common misconception is that the data required by a fixed lookup table is defined by its content and declared in a Dim statement, with its data filled at runtime. This implies that an array ( in RAM memory ) is empty initially and populated during initialization, leading to data duplication and wasted memory resources.

Correct Implementation

A fixed lookup table is a set of data (bytes, words, etc.) stored in the PROGMEM memory. The correct implementation involves using the TABLE and READTABLE commands: * Definition: TABLE tablename…​ data…​ END TABLE * Reading: READTABLE There is no DIM in the definition process, and the data is part of the hex file, not filled at runtime.

Memory Efficiency

Storing data in PROGMEM ensures that there is only one copy of the data, avoiding duplication. Copying data to an array is redundant as reading the table can replace the array.

Practical Solutions

Using TABLE - END TABLE is the simplest way to handle data. For data sets smaller than the EEPROM in the chip, load the table directly to EEPROM and use READTABLE to read the data.

Advanced Techniques

  1. PROGMEM Page Size Constraint: On the 16F, TABLE-END TABLE is constrained by PROGMEM page size (2048 items).
  2. EEPROM Storage: Use EEPROM .. END EEPROM and a direct method like PROGREAD. Data is stored in EEPROM, constrained by its size and typically byte values.
  3. Direct PROGMEM Storage: Use DATA .. END DATA and PROGREAD. Data is stored in PROGMEM, constrained by unused PROGMEM size and typically word values (max 0x3FFF for 16F chips).

Arrays in Embedded Systems

An array is a special type of variable that can store multiple values, addressed individually using an index. Arrays can be bytes, longs, integers, or words, and are held in RAM. Loading an array can be done element by element or all at once.

Comparison of Methods

Using arrays can be costly in terms of RAM and PROGMEM. The following examples illustrate the difference:

Using an Array

64 words Progmem / 13 bytes RAM

        #CHIP 18F2550
        #option Explicit

        Dim myResult, myIndex as Byte

        // Using an array 64 words Progmem / 13 bytes RAM
        Dim myArray(10)
            myArray = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
        For myIndex = 1 to 10
            myResult =  myArray(myIndex)
        Next

Using a Table

56 words Progmem / 2 bytes RAM

        #CHIP 18F2550
        #option Explicit

        Dim myResult, myIndex as Byte

        // Using a table 56 words Progmem / 2 bytes RAM
        For myIndex = 1 to 10
            ReadTable myTable, myIndex, myResult
        Next

        Table myTable
            1, 2, 3, 4, 5, 6, 7, 8, 9, 10
        End Table

Usage

The ReadTable method provides data set capabilities to chips with limited RAM, using fewer resources and offering faster performance. Advanced techniques and proper understanding of memory usage can significantly optimize embedded system performance.

Notes

  • A byte array is handled similarly to a string, which can be resource-intensive.

By following these guidelines, developers can efficiently implement lookup reference tables in embedded systems, optimizing memory usage and performance.

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