The Dreamcast uses 32-bit logical addresses, but the physical address range is only 29-bit. When the built-in MMU is active, the mapping between logical and physical address is basically left up to the programmer (see the SH4 Hardware Manual for details), however the normal mode of operation in the Dreamcast is to have the MMU disabled, in which the mapping is fixed as described below.

When the MMU is disabled, the 32-bit logical addresses have the following format:

31 30 29 28 ... 0 P ALT NC Physical address

In addition to the physical address, the following modifier bits are used:

P - Privileged Access

Addresses with the P bit set can only be accessed when the CPU is running in privileged mode, that is when the MD bit in the status register is set. (An exception to this is the Store Queue though.) P inhibits the MMU, and enables the ALT and NC bits. If P is not set, ALT and NC are ignored.

ALT - Alternate Address Space

Setting this bit gives an alternate mapping of the physical address bits. If NC is also set, the builtin CPU I/O registers are selected. If NC is not set, MMU translation is selected (if MMU is enabled) even though P is set.

NC - Not Cacheable

Setting this bit circumvents the data cache, which is useful for accessing external hardware registers.

The following table show the possible combinations:

P ALT NC Address range Description Area name 0 X X 0x00000000-0x7FFFFFFF (Physical address space repeats 4 times) Address translation through MMU (if enabled). Can be accessed in both User and Privileged mode. U0/P0 1 0 0 0x80000000-0x9FFFFFFF Privileged mode only. P1 1 0 1 0xA0000000-0xBFFFFFFF Privileged mode only, no cache. P2 1 1 0 0xC0000000-0xDFFFFFFF Privileged mode only. Address translation through MMU (if enabled). P3 1 1 1 0xE0000000-0xFFFFFFFF Privileged mode only. Internal I/O register access. No cache. P4

With exception of the P4 area, the different logical address areas share the same physical address space. This address space (of 29 bits) is divided into eight different areas of 64MB size each.

Physical address Contents Name 0x00000000-0x03FFFFFF Boot ROM (2MB) Flash ROM (256K) Hardware registers Area 0 0x04000000-0x07FFFFFF Video RAM (8MB) Area 1 0x08000000-0x0BFFFFFF Area 2 0x0C000000-0x0FFFFFFF System RAM (16MB) Area 3 0x10000000-0x13FFFFFF Tile accelerator command input Area 4 0x14000000-0x17FFFFFF Expansion (modem) port Area 5 0x18000000-0x1BFFFFFF Area 6 0x1C000000-0x1FFFFFFF Internal I/O registers (same as P4) Area 7

Accesses to Area 7 require the CPU to be running in privileged mode even if the access is through U0.

To be described later...

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Dreamcast Programming by Marcus Comstedt
Last modified: Sat Nov 18 19:41:50 MET 2000