Dynamic Area Flags
Programmer's Reference Manuals
» Part 15 – SWI Calls
» OS
» OS_DynamicArea
» OS_DynamicArea 0
» Flags
| Bit | Meaning |
|---|---|
| 0-3 | Access privileges to be given to each page in the area (same format as for OS_ReadMemMapEntries) |
| 4 | Set if area is not bufferable by chips (see notes) |
| 5 | Set if area is not cacheable by chips (see notes) |
| 6 | If set, area is doubly mapped (reserved), otherwise area is singly mapped |
| 7 | 0 =>area may be dragged by the user in Task Manager window (has red bar) |
| 1 =>area may not be dragged by the user in Task Manager window (has green bar) | |
| 8 | 0 =>area does not require specific physical pages (ie R1 is undefined on entry to the PreGrow? and PostGrow? handlers) |
| 1 =>area may require specific physical pages (ie R1 points at a page block on entry to the PreGrow? and PostGrow? handlers) | |
| 9 | 1 =>area is Shrinkable 1 |
| 10 | 1 =>area is Sparse 1 |
| 11 | 1 =>area is bound? to client application 1 |
| 12-14 | Cache policy 2 |
| 15-30 | Reserved (must be zero) |
| 31 | Virtual memory is active for dynamic area |
1 Only available on RISC OS 3.80 onwards.
2 Only available on RISC OS 5.
Notes
If bit 31 is set when the area is created then virtual memory is activated immediately. Note that no error is produced if it is not possible to start virtual memory for the new dynamic area
Advice on bits 4 & 5
Setting flags such that r4 bit 4 is 1 and bit 5 is 0 (area is cacheable but not bufferable) has different meanings depending on the operating system version. On RISC OS 5 it selects a non-merging write-through or write-back cache policy. On other operating system versions it is unsupported and must not be used.
Cache policies
The values of bits 4 and 5 alter the interpretation of the RISC OS 5 cache policy bits. The tables below also list a fallback policy for each entry; if the chosen policy isn’t available, the kernel will automatically try successive fallback policies until a supported policy is found.
CB (cacheable + bufferable) policies
| Policy | Meaning | Fallback policy |
|---|---|---|
| 0 | OS decides policy (Writeback if available, or write alloc for areas mapped by HAL) | - |
| 1 | Writethrough cacheable, read allocate. | NCB policy 2 (merging write buffer) |
| 2 | Writeback cacheable, read allocate. | CB policy 1 (writethrough) |
| 3 | Writeback cacheable, write allocate. | CB policy 2 (writeback cacheable, read allocate) |
| 4 | Alternative D-cache (XScale/StrongARM mini D-cache) | CB policy 0 (default policy) |
| 5-7 | Reserved | - |
NCB (non-cacheable + bufferable) policies
| Policy | Meaning | Fallback policy |
|---|---|---|
| 0 | OS decides buffer policy (currently, merging by default) | - |
| 1 | Non-merging write buffer. | NCNB |
| 2 | Merging write buffer. | NCB policy 1 (Non-merging write buffer) |
| 3-7 | Reserved | - |
CNB (cacheable + non-bufferable) policies
| Policy | Meaning | Fallback policy |
|---|---|---|
| 0 | OS decides cache policy (currently, writethrough by default) | - |
| 1 | Writethrough cache, non-buffered. | NCNB |
| 2 | Writeback cache, non-buffered. | CNB policy 1 (Writethrough cache, non-buffered) |
| 3-7 | Reserved | - |
NCNB (non-cacheable, non-bufferable) policies
| Policy | Meaning | Fallback policy |
|---|---|---|
| 0 | Default policy | None |
| 1-7 | Reserved | - |
See also
- Dynamic Area PostGrow?
- Dynamic Area PreGrow?
- OS_DynamicArea
- Memory Map Page Access
- Shrinkable Dynamic Areas
- Sparse Dynamic Areas