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112 lines
4.2 KiB
Markdown
112 lines
4.2 KiB
Markdown
A [_BYTE](_BYTE) variable can hold signed variable values from -128 to 127 (one byte or 8 [_BIT](_BIT)s). [_UNSIGNED](_UNSIGNED) from 0 to 255.
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## Syntax
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> [DIM](DIM) byte [AS](AS) [[_UNSIGNED]([_UNSIGNED)] [[_BYTE]]
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## Description
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* Signed _BYTE values can range from -128 to 127.
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* [_UNSIGNED](_UNSIGNED) _BYTEs can hold values from 0 to 255. [_UNSIGNED](_UNSIGNED) expands the range of positive values.
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* Can be defined in a **QB64** [_DEFINE](_DEFINE) statement using a starting letter range of variable names.
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* Also can be used in a subroutine parameter [AS](AS) _BYTE variable definitions.
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* Define a byte using the suffix %% after the variable name: variable%% = -54
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* Define an unsigned byte by adding the suffix ~%% after the variable name: variable~%% = 54
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* **When a variable has not been assigned or has no type suffix, the value defaults to [SINGLE](SINGLE).**
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**[_BIT](_BIT)**
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* The **MSB** is the most significant(largest) bit value and **LSB** is the least significant bit of a binary or register memory address value. The order in which the bits are read determines the binary or decimal byte value. There are two common ways to read a byte:
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* **"Big-endian"**: MSB is the first bit encountered, decreasing to the LSB as the last bit by position, memory address or time.
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* **"Little-endian"**: LSB is the first bit encountered, increasing to the MSB as the last bit by position, memory address or time.
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```text
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**Offset or Position: 0 1 2 3 4 5 6 7 Example: 11110000**
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---------------------------------- --------
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**Big-Endian Bit On Value:** 128 64 32 16 8 4 2 1 240
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**Little-Endian Bit On Value:** 1 2 4 8 16 32 64 128 15
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```
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> The big-endian method compares exponents of 2<sup>7</sup> down to 2<sup>0</sup> while the little-endian method does the opposite.
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**[_BYTE](_BYTE)**
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* [INTEGER](INTEGER) values consist of 2 bytes called the **HI** and **LO** bytes. Anytime that the number of binary digits is a multiple of 16 (2bytes, 4 bytes, etc.) and the HI byte's MSB is on(1), the value returned will be negative. Even with [SINGLE](SINGLE) or [DOUBLE](DOUBLE) values!
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```text
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**16 BIT INTEGER OR REGISTER**
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**AH (High Byte Bits) AL (Low Byte Bits)**
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BIT: 15 14 13 12 11 10 9 8 | 7 6 5 4 3 2 1 0
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---------------------------------------|--------------------------------------
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HEX: 8000 4000 2000 1000 800 400 200 100 | 80 40 20 10 8 4 2 1
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DEC: -32768 16384 8192 4096 2048 1024 512 256 | 128 64 32 16 8 4 2 1
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```
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> The HI byte's **MSB** is often called the **sign** bit! When all 16 of the integer binary bits are on, the decimal return is -1.
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## Example(s)
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> How negative assignments affect the _UNSIGNED value returned by a byte (8 bits).
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```vb
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DIM unsig AS _UNSIGNED _BYTE
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DIM sig AS _BYTE
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CLS
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unsig = 1
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sig = 1
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PRINT "00000001 = unsigned & signed are both" + STR$(unsig AND sig)
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unsig = 127
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sig = 127
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PRINT "&B01111111 = unsigned & signed are both" + STR$(unsig AND sig)
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unsig = 255
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sig = 255
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PRINT "&B11111111 = unsigned is" + STR$(unsig) + " but signed is " + STR$(sig)
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unsig = 254
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sig = 254
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PRINT "&B11111110 = unsigned is" + STR$(unsig) + " but signed is " + STR$(sig)
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unsig = 253
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sig = 253
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PRINT "&B11111101 = unsigned is" + STR$(unsig) + " but signed is " + STR$(sig)
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PRINT
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PRINT "The signed value needs the MSB bit for the sign."
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PRINT "The most significant bit is furthest to the left."
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```
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```text
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&B00000001 = unsigned & signed are both 1
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&B01111111 = unsigned & signed are both 127
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&B11111111 = unsigned is 255 but signed is -1
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&B11111110 = unsigned is 254 but signed is -2
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&B11111101 = unsigned is 253 but signed is -3
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The signed value needs the MSB bit for the sign.
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The most significant bit is furthest to the left.
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```
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## See Also
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* [_BIT](_BIT), [&B](&B)
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* [_DEFINE](_DEFINE), [DIM](DIM)
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* [_UNSIGNED](_UNSIGNED)
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* [_SHL](_SHL), [_SHR](_SHR)
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* [Mathematical Operations](Mathematical-Operations)
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* [Screen Memory](Screen-Memory)
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* [Variable Types](Variable-Types)
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* [Converting Bytes to Bits](Converting-Bytes-to-Bits)
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