Acorn Arcade forums: Programming: Some RISC OS memory questions
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Some RISC OS memory questions |
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ksattic (19:05 21/7/2007)
Phlamethrower (19:56 21/7/2007)
ksattic (20:15 21/7/2007)
Phlamethrower (20:30 21/7/2007)
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Simon Wilson |
Message #103576, posted by ksattic at 19:05, 21/7/2007 |
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I have survived this long without an intimate knowledge of the RISC OS memory system, and now is the time for me to ask some questions. I looked online for answers for a few hours this morning, but couldn't find the answers I need.
PCI_RAMAlloc allocates RAM from a fixed, contiguous memory pool, and will return in R0 the logical address (for application access to memory locations) and in R1 the PCI address (physical address) for use by PCI bus masters (i.e. a TV card).
malloc, when called from an application, allocates RAM from the heap and is not guaranteed to be contiguous. The address returned for the start of memory is a logical address.
Questions:
1. How do I allocate a page of memory (4096 bytes)? I would like to avoid allocating 8191 bytes and aligning the start address to the 4096 byte boundary.
2. How do I find the physical and logical addresses of this memory?
3. How do I convert from a logical to a physical address?
4. If the logical and physical address on RISC OS is the same (I'm not sure if this is the case or not), then why does memory allocated with PCI_RAMAlloc have different logical and physical addresses? |
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Jeffrey Lee |
Message #103577, posted by Phlamethrower at 19:56, 21/7/2007, in reply to message #103576 |
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1. How do I allocate a page of memory (4096 bytes)? I would like to avoid allocating 8191 bytes and aligning the start address to the 4096 byte boundary. I think the correct way of allocating pages from the system is by creating a dynamic area, as they will (I believe) always start on page boundaries, and (I assume) will use contiguous pages where possible (and therefore contiguous hardware addresses)
Unless of course you're talking about allocating a page from an area of RAM which is already yours - in which case you'd need a memory manager that supports allocations at arbitraty alignments (or just allocate a larger buffer and round up the address).
2. How do I find the physical and logical addresses of this memory?
3. How do I convert from a logical to a physical address? OS_ReadMemMapInfo, OS_ReadMemMapEntries, and OS_FindMemMapEntries.
Physical address = page number*page size.
4. If the logical and physical address on RISC OS is the same (I'm not sure if this is the case or not), then why does memory allocated with PCI_RAMAlloc have different logical and physical addresses? The logical and physical addresses aren't the same.
Note that I don't have much experience with RISC OS's memory management either, so this is probably wrong in every single way  |
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Simon Wilson |
Message #103578, posted by ksattic at 20:15, 21/7/2007, in reply to message #103577 |
Finally, an avatar!
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OS_ReadMemMapInfo, OS_ReadMemMapEntries, and OS_FindMemMapEntries.
Physical address = page number*page size. Excellent, this is exactly what I was thinking/hoping.
Note that I don't have much experience with RISC OS's memory management either, so this is probably wrong in every single way Cheers, but I think you are right.
By the way, what is the size of the PCI memory pool in RISC OS 5? I could probably get an estimate of this by allocating larger blocks until it fails. |
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Jeffrey Lee |
Message #103579, posted by Phlamethrower at 20:30, 21/7/2007, in reply to message #103578 |
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By the way, what is the size of the PCI memory pool in RISC OS 5? I could probably get an estimate of this by allocating larger blocks until it fails. The pool which PCI_RAMAlloc uses? Looking at the task manager, there's a 'PCI shared memory' dynamic area, currently sitting at just over 1MB. But it can be dragged up to 32MB, so I suspect that's the maximum size  You can probably determine this size programmatically (if a little unreliably) by using OS_DynamicArea - i.e. search for the area with the 'PCI shared memory' name and look at the size info.
If you're talking about the amount of logical address space allocated for PCI devices (i.e. the space which PCI_HardwareAddress gets logical addresses from) then I've got no idea what the maximum is. Presumably 1GB or more (RISC OS is limited to 2GB main RAM, right?) |
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Acorn Arcade forums: Programming: Some RISC OS memory questions |
