94 lines
4.4 KiB
Markdown
94 lines
4.4 KiB
Markdown
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# Memory Map
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#### Memory Map
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```
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KUSEG KSEG0 KSEG1
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00000000h 80000000h A0000000h 2048K Main RAM (first 64K reserved for BIOS)
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1F000000h 9F000000h BF000000h 8192K Expansion Region 1 (ROM/RAM)
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1F800000h 9F800000h -- 1K Scratchpad (D-Cache used as Fast RAM)
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1F801000h 9F801000h BF801000h 8K I/O Ports
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1F802000h 9F802000h BF802000h 8K Expansion Region 2 (I/O Ports)
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1FA00000h 9FA00000h BFA00000h 2048K Expansion Region 3 (whatever purpose)
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1FC00000h 9FC00000h BFC00000h 512K BIOS ROM (Kernel) (4096K max)
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FFFE0000h (KSEG2) 0.5K I/O Ports (Cache Control)
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```
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Additionally, there are a number of memory mirrors.<br/>
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#### Additional Memory (not mapped to the CPU bus)
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```
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1024K VRAM (Framebuffers, Textures, Palettes) (with 2KB Texture Cache)
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512K Sound RAM (Capture Buffers, ADPCM Data, Reverb Workspace)
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0.5K CDROM controller RAM (see CDROM Test commands)
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16.5K CDROM controller ROM (Firmware and Bootstrap for MC68HC05 cpu)
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32K CDROM Buffer (IC303) (32Kx8) (BUG: only two sectors accessible?)
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128K External Memory Card(s) (EEPROMs)
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```
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#### KUSEG,KSEG0,KSEG1,KSEG2 Memory Regions
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```
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Address Name Size Privilege Code-Cache Data-Cache
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00000000h KUSEG 2048M Kernel/User Yes (Scratchpad)
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80000000h KSEG0 512M Kernel Yes (Scratchpad)
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A0000000h KSEG1 512M Kernel No No
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C0000000h KSEG2 1024M Kernel (No code) No
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```
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Kernel Memory: KSEG1 is the normal physical memory (uncached), KSEG0 is a
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mirror thereof (but with cache enabled). KSEG2 is usually intended to contain
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virtual kernel memory, in the PSX it's containing Cache Control I/O Ports.<br/>
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User Memory: KUSEG is intended to contain 2GB virtual memory (on extended MIPS
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processors), the PSX doesn't support virtual memory, and KUSEG simply contains
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a mirror of KSEG0/KSEG1 (in the first 512MB) (trying to access memory in the
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remaining 1.5GB causes an exception).<br/>
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#### Code Cache
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Works in the cached regions (KUSEG and KSEG0).<br/>
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There are reportedly some restrictions... not sure there... eventually it is
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using the LSBs of the address as cache-line number... so, for example, it
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couldn't simultaneously memorize opcodes at BOTH address 80001234h, AND at
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address 800F1234h (?)<br/>
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#### Data Cache aka Scratchpad
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The MIPS CPU usually have a Data Cache, but, in the PSX, Sony has misused it as
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"Scratchpad", that is, the "Data Cache" is mapped to a fixed memory location at
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1F800000h..1F8003FFh (ie. it's used as Fast RAM, rather than as cache).<br/>
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There \<might\> be a way to disable that behaviour (via Port FFFE0130h or
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so), but, the Kernel is accessing I/O ports via KUSEG, so activating Data Cache
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would cause the Kernel to access cached I/O ports.<br/>
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Not tested yet, but most probably the Scratchpad can be used only for Data (ie.
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NOT for program Code?).<br/>
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#### Memory Mirrors
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As described above, the 512Mbyte KUSEG, KSEG0, and KSEG1 regions are mirrors of
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each other. Additional mirrors within these 512MB regions are:<br/>
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```
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2MB RAM can be mirrored to the first 8MB (strangely, enabled by default)
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512K BIOS ROM can be mirrored to the last 4MB (disabled by default)
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Expansion hardware (if any) may be mirrored within expansion region
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The seven DMA Control Registers at 1F8010x8h are mirrored to 1F8010xCh
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```
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The size of the RAM, BIOS, Expansion regions can be configured by software, for
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Expansion Region it's also possible to change base address, see:<br/>
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[Memory Control](memorycontrol.md)<br/>
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The Scratchpad is mirrored only in KUSEG and KSEG0, but not in KSEG1.<br/>
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#### Memory Exceptions
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```
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Memory Error ------> Misalignments
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(and probably also KSEG access in User mode)
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Bus Error ------> Unused Memory Regions (including Gaps in I/O Region)
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(unless RAM/BIOS/Expansion mirrors are mapped to "unused" area)
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```
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#### More Memory Info
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For Info on Exception vectors, Unused/Garbage memory locations, I/O Ports,
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Expansion ROM Headers, and Memory Waitstate Control, etc. see:<br/>
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[I/O Map](iomap.md)<br/>
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[Memory Control](memorycontrol.md)<br/>
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[EXP1 Expansion ROM Header](expansionportpio.md#exp1-expansion-rom-header)<br/>
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[BIOS Memory Map](kernelbios.md#bios-memory-map)<br/>
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[BIOS Memory Allocation](kernelbios.md#bios-memory-allocation)<br/>
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[COP0 - Exception Handling](cpuspecifications.md#cop0---exception-handling)<br/>
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[Unpredictable Things](unpredictablethings.md)<br/>
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