8.7 Using EDS

EDS (Extended Data Space) is an architectural concept that allows the mapping of extra RAM into the 16-bit data addressable area. This feature uses a paging scheme, mapping in 32K pages into the address range from 0x8000 to 0xFFFF (bit 15 is set). EDS is similar to PSV (Program Space Visibility) which all 16-bit devices support.

__eds__ is a compiler concept that allows this space to be used by either additional RAM or by FLASH. This is an extension of the architectural PSV (Program Space Visisbility) window and compiler's __psv__/__prog__ interpretation. PSV only facilitates the mapping of FLASH pages; EDS allows mapping RAM or Flash pages. __eds__ can be used on all 16-bit devices, even older devices that do not have any EDS memory. In this case, __eds__ can be used to access Flash.

__eds__, __psv__, and __prog__ are treated as address space qualifiers and define an access method for the compiler. These words are also used inside an address attribute to define permissible allocation.

__attribute__((space(psv))), or __attribute__((space(__psv__))), describe to the language tool where an object may be placed. Access and allocation are separated to allow access to be defined by the individual customer, if needed. Typically, an object allocated in a named address space would also be tagged with an address space qualifier. Here are some examples:

__psv__ Tau object1 __attribute__((space(psv)));

__eds__ Tau object2 __attribute__((space(psv)));

object1 is allocated somewhere in PSV (Flash), and accessed through the compiler's __psv__ mechanism; the compiler will manage the setting of the EDS page to access the object. object2 is also allocated in PSV but accessed through the more general __eds__ mechanism, also completely managed by the compiler.

Pointer declarations may also have address space qualifiers, but not be allocated in a named address space. In this case, it would represent a normal data space pointer, which is pointing to an object in one of the named address spaces:

__psv__ int *pointer_to_psv_int;

int * __psv__ psv_pointer_to_int __attribute__((space(psv)));

The way to decode these is to read outwards from the object name. The first defines an object, in data space, that points to an object in psv space. The 2nd defines an object that lives in psv space which points to an object in data space; this object needs a space attribute to get located into an appropriate named address space.