#Option UserCodeOnly

Syntax:

    #option UserCodeOnly LABEL:

This option enables minimal‑startup user mode, allowing the developer to take full control of the program’s behaviour from the very first instruction.

When enabled, the compiler omits all standard automatic startup routines normally inserted by GCBASIC. This directive is ideal for applications where the user requires absolute control over the execution environment.

The label is mandated. The label specified will be included in the ASM generated.

Behaviour:

Enabling UserCodeOnly removes or suppresses the following:

  • No automatic stack initialisation The compiler does not configure the hardware stack or related registers.
  • No global interrupt enable (sei) Interrupts remain disabled unless explicitly enabled by the user.
  • Optional removal of the standard interrupt vector table If the label provided replaces the default vector, the user must supply their own interrupt entry code.
  • No GCBASIC runtime startup code No variable initialisation, no system setup, no runtime housekeeping.
  • No automatic calls to INITSYS or other init routines All system configuration must be performed manually.
  • Reduced .ORG directives at page boundaries The compiler avoids inserting automatic page‑alignment directives, giving the user full control of memory layout.

This mode is intended for advanced users who require a bare‑metal environment.

Ideal for:

  • Bare‑metal applications Where the user wants to define every instruction executed from reset.
  • Custom bootloaders Allows precise control over memory layout, vectors, and startup flow.
  • Mixed GCBASIC + assembly projects Ensures no hidden runtime code interferes with hand‑written assembly.
  • Minimal firmware with full user control Perfect for ultra‑small, deterministic, or timing‑critical applications.

Example 1:

    'Set chip model
    #chip 16f877a

    'Enable minimal user‑mode startup
    #option usercodeonly StartHere:

StartHere:
    'User-defined reset entry point
    #asmraw[
        nop
        nop
    #asmraw]

    'Your program continues here

Example 2:

    'Set chip model
    #chip 18f452

    'UserCodeOnly with custom interrupt vector
    #option USERCODEONLY    myReset:



MyReset:
    'Manual system setup
        clrf INTCON     ; interrupts remain disabled
        clrf STATUS

MainLoop:
    goto MainLoop
Prev  Up  Next
  Home