data_types

Data Types

This section discusses the different types and sizes of data variables used by Great Cow BASIC, and how they are interpreted or handled by Great Cow BASIC methods.

The section also provides an insight of which type of variable to use and when.

What variable sizes are suported by Great Cow BASIC?

Great Cow BASIC implements support for Bit, Byte, Word, Integer and Long Variable Types, all of which are described below.

Supported variables are Bit (1 Bit), Byte (8 Bit), Word (16 Bit), Long (32 Bit). Great Cow BASIC does not support decimal numbers.

Bit is used as a Flag or a Port Pin and has two states which may be:

    ON or OFF
    TRUE or FALSE
    HIGH or LOW
    1 or 0
    SET or RESET

other complementary states depending on how your application interprets and handles the data.

Byte is the most common size in 8 Bit devices and could represent a Number, an ASCII Character, a Port, two Nibbles (as used by Hex or BCD number systems), an Internal Register, an 8 bit Variable or any user defined collection of to eight Bits such as a group of flags.

Word is normally used for its Numeric value. 16 Bits will allow it to store Numbers from Zero to 65535 which is large enough to store the product of any two 8 bit Bytes without overflowing.   However, it is not confined to being used as a numeric value.   A Word may be used in any manner that your application needs depending on how it interprets the 16 Bits of data. Examples may be a memory address or a data pointer.

• Note: The Word size of a device (as opposed to the Word Type above) is a representation of the number of Bits that it can manipulate simultaneously by the chip.    The number of Bits for PIC and AVR Microcontrolers supported by Great Cow BASIC are 8 Bits and so they are considered to have an 8 Bit Word.*

Long is for situations where Values exceeding 65535 have to be handled and has a range of zero to 4.29 Billion.  It is rarely used in 8 Bit devices but is invaluable on the rare occasions that it is needed.    The Millis function for example uses the Long Data Type to handle time periods of up to 50 days.

All of the above can be considered to be Integer Values of varying magnitude as they can hold non Fractional Positive Whole Numbers, but try not to confuse Integer Values with the Integer Variable Type, they are complementary but separate concepts as you will see below.

An integer is a whole number (not a fractional number) that can be Positive, Negative, or Zero.

In your application there may be a need to be able represent Negative Numbers in our variables and that is where the Great Cow BASIC Integer Variable Type is useful. An Integer Variable is similar to the Word Variable as they are both 16 bits.    The difference how the Great Cow BASIC compiler interprets the data bits that it contains.

The compiler will treat a Word Variable Type as a Variable that can store the values 0 < 65535 but it will see the Integer Variable Type as a Variable that can store values of -32768 < 0 <32767.

Variable size

Each type of variable is defined in various bit lengths, in this case Great Cow BASIC these are:

    Byte                 8 Bit
    Integer             16 Bit
    Word                16 Bit
    Long                32 Bit

All four of the above are number types are true Integers.   In that they are representations of a integer non fractional number as follows:

    8  Bit - an 8 Bit number can be in the range of 0 to 255
    16 Bit - a 16 Bit number can be in the range of 0 to 65535
    32 Bit - a 32 Bit number can be in the range of 0 to 43294967295

But, they can only represent positive numbers.   In Mathematics there is a need for an Integer that can be Positive, Negative, or Zero.   Note that Zero is always a Positive Whole Number.

Two’s Complement

To take the Two’s Complement of a number it is inverted then incremented:

        MyVar = NOT MyVar + 1

The increment, of adding 1, has two effects, it avoids the possible creation of a negative zero as a value of 1000000 would be seen as -128 and it allows subtraction to be achieved through addition.

If MyVar contained a value of 1 the 8 Bit representation would be:

    00000001

The NOT will make it

    11111110

Note that the Most significant Bit is now 1 so as a signed value it is negative.

The increment will result in a value of:

    11111111

So Minus one using an 8 Bit representation in Two’s Complement notation is 11111111

Let’s test it by adding -1 to plus 3

    11111111    -1
    00000011 +   3
    ==============
    00000010     2

We have successfully subtracted 1 from 3 by adding Minus 1 to 3 and obtaining a result of 2.

Notice that while a Byte is normally used to represent 0 < 255 by making the MSB (Most Significant Bit) into a sign bit the maximum value is now 127.   A signed 8 Bit integer can represent numbers in the range -128 < 0 < 127.   That is still 256 values including Zero but they can now be Negative or Positive numbers.

The benefit of the two’s complement method is that it works for any size of variable:

    MyByte = NOT MyByte +1
    MyWord = NOT MyWord +1
    MyLong = NOT MyLong +1

All of the above will result in a Negated version of the original contents.

But not all, in fact relatively few, methods of a microcontroller require negative values so in situations where negative values are not required the loss of half of the magnitude of a Byte or Word can be significant. That is why it is necessary to be able to specify if a value is Signed or Unsigned, that is if the MSB is the sign bit or part of the value.

It is obviously important from the above that the user program ds need to know what sort of data to expect as a value of 0xFF could be considered to be both 255 and -1 depending on the interpretation of the variable.    That is why it is important to have Signed and Unsigned Data Types so that the compiler can decide how to handle or interpret the contents.   As show above in Great Cow BASIC those types are referred to as Integer and Word respectively.

Summary

Great Cow BASIC implements support for Bit, Byte, Word, Integer and Long Variable Types, all of which are described above.

And, that negative numbers are represented as two’s complement.