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The _MEM variable type can be used when working with memory blocks. It has no variable type suffix.




Variable TYPE:

  • Memory DOT values are actually part of the built in memory variable type in QB64. The following TYPE is built in:

TYPE memory_type OFFSET AS _OFFSET 'start location of block(changes with byte position) SIZE AS _OFFSET 'size of block remaining at offset(changes with position) TYPE AS _OFFSET 'type description of variable used(never changes) ELEMENTSIZE AS _OFFSET 'byte size of values inside the block(never changes) IMAGE AS LONG 'the image handle used when _MEMIMAGE(handle) is used SOUND AS LONG 'the sound handle used when _MEMSOUND(handle) is used END TYPE The above TYPE is for clarification purposes only. It doesn't need to be pasted in a program to use _MEM.


  • The _MEM type contains the following read-only elements where name is the _MEM variable name:
name.OFFSET is the current start position in the memory block AS _OFFSET. Add bytes to change position.
name.SIZE is the remaining size of the block at current position in bytes AS _OFFSET
name.TYPE is the type (represented as bits combined to form a value) AS _OFFSET:

.TYPE values

  • [bit 0] 1* byte types (_BYTE)
  • [bit 1] 2* byte types (INTEGER)
  • [bit 2] 4* byte types (LONG or SINGLE)
  • [bit 3] 8* byte types (DOUBLE or _INTEGER64)
  • [bit 4] 16* byte types (reserved for future use)
  • [bit 5] 32* byte types (_FLOAT)
  • [bit 6] 64* byte types (reserved for future use)
  • [bit 7] 128 = integer types (_BYTE, INTEGER, LONG, _INTEGER64) (added to *)
  • [bit 8] 256 = floating point types (SINGLE, DOUBLE, _FLOAT) (added to *)
  • [bit 9] 512 = STRING types (fixed length or variable length)
  • [bit 10] 1024 = _UNSIGNED types (added to *+128)
  • [bit 11] 2048 = pixel data usually from _MEMIMAGE (added to 1+128+1024 for 256 color screens, or 2+128+1024 for text screens, or 4+128+1024 for 32-bit color screens)
  • [bit 12] 4096 = _MEM TYPE structure (NOT added to 32768)
  • [bit 13] 8192 = _OFFSET type (added to 4+128+[1024] or 8+128+[1024] or future_size+128+[1024])
  • [bit 14] 16384 = data created/defined by _MEMNEW(size) or _MEMNEW(offset,size)
  • [bit 15] 32768 = a custom, user defined type (ie. created with TYPE name ... END TYPE)
  • [bit 16] 65536 = an array of data (added to other type values defining the array's data type)

Note: If a future integer, float or other type doesn't have a size that is 1,2,4,8,16,32,64,128 or 256 it won't have a size-bit set.

  • Note: _OFFSET values cannot be cast to other variable types reliably. _MEM is a reserved custom variable type.
  • _MEM cannot reference variable length STRING variable values. String values must be designated as a fixed-length string.


Example 1: Demonstration of .IMAGE to determine an image's dimensions, .TYPE to verify the type and _MEMEXISTS to check image has not been freed

SCREEN _NEWIMAGE(500, 500, 32) i = _LOADIMAGE("qb64_trans.png", 32) _PUTIMAGE (0, 0), i DIM m AS _MEM m = _MEMIMAGE(i) 'try uncommenting the following line and see what happens '_MEMFREE m t = m.TYPE IF t AND 2048 THEN PRINT "this is/was an image" IF _MEMEXISTS(m) THEN 'check if memory m is still available PRINT t AND 7; "bytes per pixel" PRINT "image handle "; m.IMAGE PRINT "image width"; _WIDTH(m.IMAGE) PRINT "image height"; _HEIGHT(m.IMAGE) ELSE PRINT "Memory already freed!" END IF END IF

Example 2: Converts the current destination SCREEN 13 image memory altered by PSET to a STRING value. SCREEN 13 only.

SCREEN 13 PSET (0, 0), ASC("H") 'top left corner of screen PSET (1, 0), ASC("E") PSET (2, 0), ASC("L") PSET (3, 0), ASC("L") PSET (4, 0), ASC("O") DIM m AS _MEM m = _MEMIMAGE(0) 'copy the screen memory to m x1$ = _MEMGET(m, m.OFFSET, STRING * 5) 'get at block start position LOCATE 2, 1:PRINT LEN(x1$) 'prints 5 bytes as size is STRING * 5 PRINT x1$ 'prints HELLO as ASCII character values PRINT m.OFFSET; m.SIZE; m.ELEMENTSIZE _MEMFREE m

5 HELLO 5448320 6400 1

Explanation: When a numerical _BYTE value is converted to a STRING, each byte is converted to an ASCII character. The QB64 IDE will capitalize _MEM dot values.

m.SIZE = 320 * 200 = 6400 bytes m.ELEMENTSIZE = 1 byte

Example 3: Using _MEM to convert _OFFSET to _INTEGER64.

DIM x AS INTEGER DIM m AS _MEM m = _MEM(x) PRINT m.OFFSET PRINT ConvertOffset(m.OFFSET) FUNCTION ConvertOffset&& (value AS _OFFSET) $CHECKING:OFF DIM m AS _MEM 'Define a memblock m = _MEM(value) 'Point it to use value $IF 64BIT THEN 'On 64 bit OSes, an OFFSET is 8 bytes in size. We can put it directly into an Integer64 _MEMGET m, m.OFFSET, ConvertOffset&& 'Get the contents of the memblock and put the values there directly into ConvertOffset&& $ELSE 'However, on 32 bit OSes, an OFFSET is only 4 bytes. We need to put it into a LONG variable first _MEMGET m, m.OFFSET, temp& 'Like this ConvertOffset&& = temp& 'And then assign that long value to ConvertOffset&& $END IF _MEMFREE m 'Free the memblock $CHECKING:ON END FUNCTION

Explanation: The above will print two numbers which should match. These numbers will vary, as they're representations of where X is stored in memory, and that position is going to vary every time the program is run. What it should illustrate, however, is a way to convert _OFFSET to _INTEGER64 values, which can sometimes be useful when trying to run calculations involving mem.SIZE, mem.TYPE, or mem.ELEMENTSIZE.

See also

Keyword Reference - Alphabetical
Keyword Reference - By Usage
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