OS_ValidateAddress (changes)

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Programmer's OS Reference SWI Manuals Calls

» OS_ValidateAddressPart 15 – SWI Calls

OS_ValidateAddress

(SWI &3A)

Use

The purpose of this call is to check that a given address range is valid, i.e. that it maps to some kind of RAM.

Notes

This call does not check that the memory actually exists, only that it ought to exist. If you have used OS_SetMemMapEntries, then this call will not work as expected. This call also does not report any access information – no guarantee is made that you will be able to read or write to the memory in your current CPU mode.

The behaviour of OS_ValidateAddress has changed significantly between different OS versions.

Archimedes

Prior to RISC OS 3.5, the following areas were considered valid:

• Any logical address (0-32MB) which has RAM mapped to it
• The second mapping of screen memory at the start of the physical mapping of RAM
• (In RISC OS 3) anything reported as valid by Service_ValidateAddress

RISC OS 3.5

From RISC OS 3.5, the following areas were considered valid:

• Any dynamic area
• Several special kernel regions – zero page workspace, processor stacks, etc.
• Screen memory
• Anything reported as valid by Service_ValidateAddress

However note that this version of Service_ValidateAddress does not correctly detect valid regions of Sparse Dynamic Areas. Additionally, not all special regions are reported – e.g. the CAM, page tables, kernel buffers used with long command lines, RISC OS 5 HAL workspace, etc.

RISC OS Select

Under RISC OS Select the call was reimplemented as a call to OS_Memory 24. Physically mapped regions will not be reported as valid, however Service_ValidateAddress can be used to override this. The LegacyScreen module uses this technique to mark screen memory as valid. All other regions reported by OS_Memory 24 are considered valid, e.g. system dynamic areas such as the ROM dynamic area. Special code also exists to mark zero page as valid.

RISC OS 5.21

In RISC OS 5.21 the call was also changed to be implemented via OS_Memory 24. The OS considers the following locations as valid:

• Any non-physically mapped memory which is completely user or privileged readable or writable
• Any completely physically mapped memory which is also completely user readable & writable (i.e. screen memory on some systems)
• Anything reported as valid by Service_ValidateAddress.

Under RISC OS 5 no special code is required to mark zero page as valid, as zero page is one of the areas which OS_Memory 24 will report on.

See also

• OS_Memory 24
• OS_SetMemMapEntries
• Service_ValidateAddress