26 lines
1.4 KiB
Plaintext
26 lines
1.4 KiB
Plaintext
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The HP300 version uses a kernel which is loaded into the range 0xfffxxx... but
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is linked for range 0x000.. This causes the problem that with switching the
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MMU to the mapped kernel, the PC becomes invalid. The HP version solves the
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problem by mapping 1:1 the last page of physical memory into virtual memory.
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On the Amiga, there *is* memory at PA 0x0, so we might as well use it, causes
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much less grief and weirdness in locore.s. However, since memory down there
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is CHIPMEM, inherently slower than FASTMEM, and badly needed for framebuffer
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storage space, we'll copy the kernel over to the first bank of FASTRAM, and
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when enabling the MMU, the kernel will execute in FASTRAM, although the PC
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stays the same.
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This strategy has the big advantage (compared to the solution in Amiga MACH)
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that we can link the kernel absolutely to VA 0, for all memory models, since
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we'll never execute the kernel in the FASTMEM space while the MMU is turned
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off.
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This strategy also means, that we don't have to relocate any addresses while
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bootstrapping the mmu!!
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Initialization of the MMU happens in amiga_init.c. This file is quite a mess,
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I have generated it trying to understand what's happening in the hp300 locore.s
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file. I think, it should be able to handle MMU initialization much cleaner
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now that we don't have to think about relocation until the MMU is enabled.
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If you need your daily bit of horror, take a look at amiga_init.c...
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