External Memory HOWTO

When your projects get bigger, the internal RAM of the P89V664 is no longer sufficient. External RAM can be used to overcome this limitation. The P89V664 used in the ZKit-51 has 2KB of on-chip external RAM.

The external RAM can be used from SDCC, in two ways

  • By using the storage class __xdata.

  • By using the large memory model.

Large Memory Model

By default the compiler uses the small memory model, where in the global variables, local variables are allocated in internal RAM. The compiler can be instructed to use the large memory model. In the large memory model all global variables and local variables are allocated in external RAM. The large memory model is specified by pass --large-model to the sdcc command.

__xdata Storage Class

In the small memory model, specific variables can be put in external RAM, using the __xdata storage class. An an array can be placed in external RAM, using the following code.

__xdata int a[10];

Enabling External RAM

By default the on-chip external RAM is disabled in the P89V664. To enable it the EXTRAM bit in the AUX (0x8E) register has to be set to 0. Please note that there is a bug in the P89V664 datasheet which indicates that the EXTRAM bit should be set to 1 to enable on-chip external RAM.

Enabling of the external RAM should be done even before main() is invoked. This is because global variable initialization is done before invoking main(). And external RAM has to enabled so that global variables placed in external RAM are properly intialized.

The SDCC startup code invokes a function named _sdcc_external_startup() before it initializes global variables. The user can implement this function and enable the on-chip external RAM within it. An implementation of _sdcc_external_startup() for the P89V664 processor is given below.

int _sdcc_external_startup()
        __sfr __at (0x8E) AUXR;
        AUXR = 0x0;

        return 0;

The return value of the function has a specific meaning. If the function returns zero, global variables are initialized next. If the function returns a non-zero value, global variables are not initialized.