DIM

Action

Dimension a variable.

 

 

Syntax

DIM var AS [XRAM/SRAM/ERAM]type [AT location/variable] [OVERLAY]

 

 

Remarks

Var	Any valid variable name such as b1, i or longname. var may also be an array : ar(10) for example. You can also use a list and created a number of variables of the same data type :  DIM A1,A2, BVAR AS BYTE. This will create 3 BYTE variables. When using a list, you may not use identifiers such as #%!&. You may also not use the optional OVERLAY.   We encourage the use of Hungarian Notation where you use a prefix instead : Dim bVar As Byte ' the b indicates a BYTE Dim iMyInt As Integer 'the i indicates an INTEGER common used prefixes : b - BYTE w - WORD dw - DWORD i - INTEGER l - LONG s - STRING dbl - DOUBLE sng - SINGLE   The IDE can show the data type of the variable when you hover the mouse above the variable name and keep the SHIFT key pressed.
Type	Bit/Boolean, Byte, Word, Integer, Long, Dword, Single, Double or String
XRAM	Specify XRAM to store variable into external memory
SRAM	Specify SRAM to store variable into internal memory (default)
ERAM	Specify ERAM to store the variable into EEPROM
OVERLAY	Specify that the variable is overlaid in memory.
location	The address or name of the variable when OVERLAY is used.

A string variable needs an additional length parameter:

Dim s As XRAM String * 10

 

In this case, the string can have a maximum length of 10 characters. Internally one additional byte is needed to store the end of string marker. Thus in the example above, 11 bytes will be used to store the string.

 

Note that BITS can only be stored in internal memory.

 

You may also specify IRAM. IRAM is the place in memory where the registers are located : absolute address 0 - 31. BASCOM uses most of these addresses, depending on the instructions/options you use. For a $TINY chip it makes sense to use IRAM since there is NO SRAM in most tiny AVR chips (TINY15 for example). You may also use to IRAM to overlay registers in memory.

 

See also Memory usage

SCOPE

The scope for DIM is global. So no matter where you use the DIM statements, the variable will end up as a global visible variable that is visible in all modules, procedures and functions.

When you need a LOCAL variable that is local to the procedure or function, you can use LOCAL.

Since LOCAL variables are stored on the frame, it takes more code to dynamic generate and clean up these variables.

 

 

AT

The optional AT parameter lets you specify where in memory the variable must be stored. When the memory location already is occupied, the first free memory location will be used. You need to look in the report file to see where the variable is located in memory.

 

 

OVERLAY

The OVERLAY option will not use any variable space. It will create a sort of phantom variable.

 

Dim x as Long at $60 'long uses 60,61,62 and 63 hex of SRAM
 
Dim B1 As Byte At $60 Overlay                     '$60 is the same as &H60
Dim B2 As Byte At $61 Overlay
 

B1 and B2 are no real variables! They refer to a place in memory. In this case to &H60 and &H61. By assigning the phantom variable B1, you will write to memory location &H60 that is used by variable X.

So to define it better, OVERLAY does create a normal usable variable, but it will be stored at the specified memory location which could be already be occupied by another OVERLAY variable, or by a normal variable.

 

Take care with the OVERLAY option. Use it only when you understand it.

 

 

You can also read the content of B1:

Print B1

 

This will print the content of memory location &H60.

 

By using a phantom variable you can manipulate the individual bytes of real variables.

 

 

Overlay example 2

Dim L as Long at &H60
Dim W as Word at &H62 OVERLAY

 

W will now point to the upper two bytes of the long.

 

Overlay example 3

 

Following you find the Bascom-AVR Simulator Memory status when you run the following example in Bascom-AVR Simulator. This example is intended to be used with the simulator. You need to uncomment the $sim when you want to test it on an real AVR.

 

Strings need an additional byte (Null termination). So you need an overlay of 8 bytes when you overlay a string with 7 bytes.

$regfile = "m644pdef.dat"
$crystal = 4000000
$hwstack = 60
$swstack = 60
$framesize = 60 'frame space can grow rapid when using it on variables with a big size (strings)
 
$baud = 9600
 
$sim '$sim to use this example in Bascom-AVR simulator
 
 
Print "-------------------------"
 
Dim Array(5) As Byte
Dim My_string As String * 4 At Array Overlay
Dim K As Byte
 
K = 1
 
 
My_string = "Test"
 
'---> 4 ASCII but 5 Bytes because of 0 Termination of String which is another byte
'This is how it will be stored in SRAM
' Array(1) Array(2) Array(3) Array(4) Array(5)
' +--------+--------+--------+--------+--------+
' | T | e | s | t | 00 |
' +--------+--------+--------+--------+--------+
 
Print Chr(array(1))
Print Chr(array(2))
 
Print "-------------------------"
 
 
Dim Teststring As String * 5
Dim Ar(6) As Byte At Teststring Overlay
Dim J As Byte
J = &H03
 
Ar(5) = 47
 
Teststring = "Hello"
 
' ---> 5 ASCII but 6 Bytes because of 0 Termination of String
'This is how it will be stored in SRAM
' Ar(1) Ar(2) Ar(3) Ar(4) Ar(5) Ar(6)
' +--------+--------+--------+--------+--------+--------+
' | H | e | l | l | o | 00 |
' +--------+--------+--------+--------+--------+--------+
 
For K = 1 To 5
Print Chr(ar(k)) ;
Next
Print
 
K = 1
 
Print "-------------------------"
 
 
 
Dim My_word As Word
Dim Low_byte As Byte At My_word Overlay
Dim High_byte As Byte At My_word + 1 Overlay
 
Low_byte = &B0000_1111
High_byte = &B1111_0000
 
'This is how it will be stored in SRAM
' <-------my_word-------->
' +-----------+----------+
' | Low_byte |High_byte |
' +-----------+----------+
 
 
'But when you print it with print bin(Variable) you will see it as
 
' <-------my_word-------->
' 11110000 00001111
' +-----------+----------+
' | High_byte |Low_byte |
' +-----------+----------+
 
 
Print "My_word = " ; Bin(my_word)
 
Print "-------------------------"
 
Dim My_long_1 As Long
Dim Byte_1 As Byte At My_long_1 Overlay
Dim Byte_2 As Byte At My_long_1 + 1 Overlay
Dim Byte_3 As Byte At My_long_1 + 2 Overlay
Dim Byte_4 As Byte At My_long_1 + 3 Overlay
 
 
Byte_1 = 1
Byte_2 = 2
Byte_3 = 3
Byte_4 = 4
 
Print Bin(my_long_1)
 
'This is how it will be stored in SRAM
' <-------my_long_1------------>
' +-------+------+------+------+
' | Byte_1|Byte_2|Byte_3|Byte_4|
' +-------+------+------+------+
 
 
'But when you print it with print bin(Variable) you will see it as
 
' <-------my_long_1------------>
' +-------+------+------+------+
' | Byte_4|Byte_3|Byte_2|Byte_1|
' +-------+------+------+------+
 
Print "-------------------------"
 
 
 
Dim My_dword As Dword At $140 'This places the my_long_2 variable at a fixed SRAM address starting at HEX 140
Dim Byte__1 As Byte At $140 Overlay 'NOTICE: because this will be stored at the specified memory location
Dim Byte__2 As Byte At $141 Overlay ' which could be already be occupied by another OVERLAY variable, or by a normal variable the
Dim Byte__3 As Byte At $142 Overlay ' compiler generate an ERROR "Address already occupied" in this case.
Dim Byte__4 As Byte At $143 Overlay
 
 
Byte__1 = 1
Byte__2 = 2
Byte__3 = 3
Byte__4 = 4
 
'This is how it will be stored in SRAM
' <----------my_dword---------->
' +-------+------+------+------+
' | Byte_1|Byte_2|Byte_3|Byte_4|
' +-------+------+------+------+
 
 
'But when you print it with print bin(Variable) you will see it as
' <----------my_dword---------->
' +-------+------+------+------+
' | Byte_4|Byte_3|Byte_2|Byte_1|
' +-------+------+------+------+
 
Print "my_dword = " ; Bin(my_dword)
 
Print "-------------------------"
 
 
 
Dim My_dword_2 As Dword
Dim My_word_2 As Word At My_dword_2 Overlay
Dim My_byte3 As Byte At My_dword_2 + 2 Overlay
Dim My_byte4 As Byte At My_dword_2 + 3 Overlay
 
 
My_word_2 = &B11111111_00000000
My_byte3 = &B00000011
My_byte4 = &B10000000
 
'This is how it will be stored in SRAM
' <--------------my_dword_2------------>
' +---------+--------+--------+--------+
' | my_word_2 |my_byte3|my_byte4|
' +---------+--------+--------+--------+
 
 
'But when you print it with print bin(Variable) you will see it as
' <--------------my_dword_2------------>
' +---------+--------+--------+--------+
' | my_byte4|my_byte3| my_word_2 |
' +---------+--------+--------+--------+
 
Print Bin(my_dword_2)
 
 
Print "-------------------------"
 
 
' Now we examine the Null terminator in Strings
 
Dim My_date(11) As Byte '8 strings + 3 Null terminator = 11 Byte
Dim Day As String * 2 At My_date(1) Overlay
Dim Null_terminator As Byte At My_date(1) + 2 Overlay 'Null terminator
Dim Month As String * 2 At My_date(1) + 3 Overlay
Dim Null_terminator_2 As Byte At My_date(1) + 5 Overlay 'Null terminator
Dim Year As String * 4 At My_date(1) + 6 Overlay
Dim Null_terminator_3 As Byte At My_date(1) + 10 Overlay 'Null terminator
 
 
Day = "16"
Month = "11"
Year = "2011"
 
Print "Day= " ; Day
Print "Month= " ; Month
Print "Year= " ; Year
 
 
 
'For example the print function use the Null Terminator to check the end of the string
'When we set now the Null_terminator to "/" (forward slash) instead of 0 then the print function print until a Null terminator is recognised
Null_terminator = 47 '47 = "/" (forward slash) '
 
Print Day 'This will now print "16/11" because the first Null terminator will be found after the "11"
 
 
 
End 'end program

Using variable name instead of address

As variables can be moved though the program during development it is not always convenient to specify an address. You can also use the name of the variable :

 

DIM W as WORD

Dim B as BYTE AT W OVERLAY

 

Now B is located at the same address as variable W.

 

For XRAM variables, you need additional hardware : an external RAM and address decoder chip.

 

 

ERAM

For ERAM variables, it is important to understand that these are not normal variables. ERAM variables serve as a way to simple read and write the EEPROM memory. You can use READEEPROM and WRITEEEPROM for that purpose too.

 

To write to an ERAM variable you have to use an SRAM variable as the source : eramVAR= sramVAR

To read from an ERAM variable you have to use an SRAM variable as the targer : sramVAR=eramVAR

Both variables need to be of the same data type. So when writing to an ERAM double, the source variable need to be of the double type too.

ERAM can be assigned with a numeric value too : eramVAR= 123

 

You can not use an ERAM variable as you would use a normal variable.

 

Also keep in mind that when you write to ERAM, you write to EEPROM, and that after 100.000 times, the EEPROM will not erase properly.

 

Dim b as byte, bx as ERAM byte

B= 1

Bx=b ' write to EEPROM

B=bx ' read from EEPROM

 

 

Xmega

The XMEGA need an additional configuration command : CONFIG EEPROM = MAPPED, in order to use ERAM.

 

 

Size

The maximum size of an array depends on the available memory and the data type. The XMEGA supports up to 8 MB of external memory. BASCOM supports this but the implementation is still considered BETA. It should not be used for production. The only thing you need to do to activate the big memory is to specify the size with $XRAMSIZE.

For example : $XRAMSIZE=8000000 will tell the compiler that you use 8 MB of external memory.

Additional registers must be set to pass the 24 bit address. This will create more code.

There is only one restriction : you can/may not pass variables located in the external memory to a sub or function.

The compiler will always pass a word address and does not support to pass the additional byte.

 

Arrays

An array is a sequential collection of elements with the same data type. Till version 2077, arrays could have only 1 index or dimension. But in 2078 this has been changed and while there are no technical limits for unlimited indexes, the limit has been set to 5. This means that you can create a variable array like :   Dim ar(5,10,5) As Byte

This will create a BYTE variable named AR, and it has 3 indexes. Each index requires space, in this sample the amount of bytes required would be : 5 * 10 * 5 = 250 * lengthOfByte = 250 bytes.

For a WORD, which uses 2 bytes, the required space would have been 250 * 2 = 500 bytes.

While using multiple indexes might be a nice feature, it comes with a penalty : the processor need to calculate the address in memory based on the indexes. The more indexes you add, the more calculations/code is required.

When you use a single index, the old calculation method is used. When using multiple indexes, a new method is used which calls array calculator code in mcs.lib.

As mentioned, the maximum number of indexes is 5 so : Dim ar(5,5,5,10,10) As Byte would work.

Multiple indexes is a new feature in 2078. The simulator does not support this option yet, so for the simulator, only 1 array exists.

 

Lets see how memory is organized when using multiple indexes. For the sample we use an array of 5x3 bytes.

Dim ar(5,3) as byte.

This gives us the following possible index values :

1,1    1,2    1,3

2,1    2,2    2,3

3,1    3,2    3,3

4,1    4,2    4,3

5,1    5,2    5,3

 

Since the memory of the processor is linear, we have 15 cells.

 

Address	Cell
n	1,1
n+1	1,2
n+2	1,3
n+3	1,4
n+4	1,5
n+5	2,1
n+6	2,2
n+7	2,3
n+8	2,4
n+9	2,5
n+10	3,1
n+11	3,2
n+12	3,3
n+13	3,4
n+14	3,5

 

 

Arrays start with element 1 by default. Thus DIM ar(5) will create 5 elements and the first element is ar(1).

Some times it is more convenient to start with element 0. For this you can use the CONFIG BASE=0 option.

When CONFIG BASE is set to 0, and not the default 1, the first element will be 0 : DIM ar(5), will make ar(0) the first element, and ar(4) the last element.

 

 

Size

The maximum size of an array depends on the available memory and the data type. The XMEGA supports up to 8 MB of external memory. BASCOM supports this but the implementation is still considered BETA. It should not be used for production. The only thing you need to do to activate the big memory is to specify the size with $XRAMSIZE.

For example : $XRAMSIZE=8000000 will tell the compiler that you use 8 MB of external memory.

Additional registers must be set to pass the 24 bit address. This will create more code.

There is only one restriction : you can/may not pass variables located in the external memory to a sub or function.

The compiler will always pass a word address and does not support to pass the additional byte.

See Also

CONST , LOCAL , Memory usage

 

 

Example

'-----------------------------------------------------------------------------------------
'name : dim.bas
'copyright : (c) 1995-2005, MCS Electronics
'purpose : demo: DIM
'micro : Mega48
'suited for demo : yes
'commercial addon needed : no
'-----------------------------------------------------------------------------------------
 
$regfile = "m48def.dat" ' specify the used micro
$crystal = 4000000 ' used crystal frequency
$baud = 19200 ' use baud rate
$hwstack = 32 ' default use 32 for the hardware stack
$swstack = 10 ' default use 10 for the SW stack
$framesize = 40 ' default use 40 for the frame space
 
Dim B1 As Bit 'bit can be 0 or 1
Dim A As Byte 'byte range from 0-255
Dim C As Integer 'integer range from -32767 - +32768
Dim L As Long
Dim W As Word
Dim S As String * 11 'length can be up to 11 characters
 
'new feature : you can specify the address of the variable
Dim K As Integer At &H120
'the next dimensioned variable will be placed after variable s
Dim Kk As Integer
 
 
'Assign bits
B1 = 1 'or
Set B1 'use set
 
'Assign bytes
A = 12
A = A + 1
 
'Assign integer
C = -12
C = C + 100
Print C
 
W = 50000
Print W
 
'Assign long
L = 12345678
Print L
 
'Assign string
S = "Hello world"
Print S
End

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