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## 11. Interrupt Service Routines

SDCC allows interrupt service routines to be coded in C, with some extended keywords.

    void timer_isr (void) interrupt 2 using 1
    {
    ..
    }

The number following the 'interrupt' keyword is the interrupt number this routine will service. The compiler will insert a call to this routine in the interrupt vector table for the interrupt number specified. The 'using' keyword is used to tell the compiler to use the specified register bank (8051 specific) when generating code for this function. Note that when some function is called from an interrupt service routine it should be preceded by a #pragma NOOVERLAY (if it is not reentrant) . A special note here, int (16 bit) and long (32 bit) integer division, multiplication & modulus operations are implemented using external support routines developed in ANSI-C, if an interrupt service routine needs to do any of these operations then the support routines (as mentioned in a following section) will have to recompiled using the --stack-auto option and the source file will need to be compiled using the --int-long-rent compiler option.

If you have multiple source files in your project, interrupt service routines can be present in any of them, but a prototype of the isr MUST be present in the file that contains the function 'main'.

Interrupt Numbers and the corresponding address & descriptions for the Standard 8051 are listed below. SDCC will automatically adjust the interrupt vector table to the maximum interrupt number specified.

    Interrupt #         Description           Vector Address
       0                External
     0            0x0003
       1                Timer 0               0x000B
       2
                    External 1            0x0013
       3                Timer 1               0x001B

       4                Serial                0x0023

If the interrupt service routine is defined without a register bank or with register bank 0 (using 0), the compiler will save the registers used by itself on the stack (upon entry and restore them at exit), however if such an interrupt service routine calls another function then the entire register bank will be saved on the stack. This scheme may be advantageous for small interrupt service routines which have low register usage.

If the interrupt service routine is defined to be using a specific register bank then only "a","b" & "dptr" are save and restored, if such an interrupt service routine calls another function (using another register bank) then the entire register bank of the called function will be saved on the stack. This scheme is recommended for larger interrupt service routines.

Calling other functions from an interrupt service routine is not recommended avoid it if possible.

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