Fundamentals Overview IDE (Editor) Syntax Variables Data Types Declarations Scope Operators

Standard Functions Arithmetic Trigonometry Strings Arrays Date/Time Flow Control Procedures Printing Files Folders

Additional Capabilities Keyboard Sound Graphics Concepts Screen (text) Screen (drawing) Gaming (sprites) Event Trapping Errors System Calls

System Calls
QBasic includes a fairly wide range of features which allow it to access/edit information in system memory and to transfer control to other programs.

Environmental	environ, environ$
String Space	fre
Port Access	inp, out
Memory Access	peek, poke
App Execution	shell, chain, common, run
Pointers	varptr, varptr$, varseg
Assembly Language	call absolute

System Calls Functions Reference
Here's a quick reference of the available system call functions, in alphabetical order.

• call absolute - transfers control to a machine-language procedure

      call absolute (varlist, offset%)
      

  The address% is relative to the current segment address as set by DEF SEG. Allowed values are 0-65535. byte% is a value of 0-255.

• chain - transfer control to another QBasic program

      chain filespec$
      chain "c:\temp\test.bas"
      

  Note: chain does not close open files

• common - defines global variables to be shared throughout a program or between chained programs

      common shared a$, b%, c#          # 3 variables made COMMON
      common a as string, b as integer  # AS defines variable types
      

  Note: unless it has been declared as a static array in a preceeding DIM statement, an array variable in a COMMON statement is a dynamic array. Its dimensions must be set in a later DIM or REDIM statement.

• environ - changes/adds a DOS environment string

      environ var$    # if not present adds var$
                      # if present, changes value
      environ "path=test"   # adds/assigns Path variable
      

• environ$ - returns a DOS environment string

      result$ = environ$ var$   # returns value of var$ 
      result$ = environ$ (n%)   # returns nth environment string
      print environ$("path")    # returns value of "path"
      

  Note: The environmental variable string must be of one of two forms:

      var$=string$
      var$ string$
      

  Note: changes made by environ are temporary. QBasic erases the changes when the program ends.

• fre - returns amount of available or unused memory

      result& = fre(-1)    # returns size of largest array
                             you can create
      result& = fre(-2)    # returns unused stack space
      result& = fre(n%)    # any other number returns available
                             string space
      result$ = fre("any") # compacts free string space/returns 
                             available space
      

• inp - returns a byte from a hardware I/O port

      result% = inp(port%)
      

• ioctl - sends control string to a device driver

      ioctl(#1, var$)   # var$ is sent to device #1
      

  The device number is that assigned using the Open statement.

• ioctl$ - returns status information from a device driver

      result$ = ioctl$(#1)   # requests status from device #1
      

  The device number is that assigned using the Open statement.

• out - sends a byte to a hardware I/O port

      out port%, data%
      

• peek - returns byte value from specified memory location

      result& = peek (address%)
      

  The address% is relative to the current segment address as set by DEF SEG. Allowed values are 0-65535.

• poke - writes a byte to a specified memory location

      poke address, byte%
      

  The address% is relative to the current segment address as set by DEF SEG. Allowed values are 0-65535. byte% is a value of 0-255.

• run - runs the current program or a specified program

      run               # executes current program at first line
      run (line%)       # executes current program at line%
      run ("c:\temp\test.bas")   # runs a QBasic source file
      

  Run closes all file and clears program memory before loading a program. Use the CHAIN program to run a program without closing open files.

• shell - suspends execution to run a DOS command or batch file

      shell var$   # var$ is name of DOS command/batch file
      

  The QBasic program continues when the DOS command or batch file completes. With no var$, a DOS window is displayed (use EXIT in the DOS window to resume your program)

• varptr - returns offset address of a variable

      result% = varptr(var$)   # returns offset address of var$
      

• varptr$ - returns string representation of variables address for use in DRAW/PLAY statements

      result$ = varptr$(var$)  # var$ contains DRAW/PLAY statements
      

• varseg - returns segment address of a variable

      result% = varseg(var$)   # returns segment address of var$
      

If you have any suggestions or corrections, please let me know.