Sbus Slots; Memory Management Unit - Sun Microsystems SPARCstation 2 Service Manual

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SBus Slots

Memory Management Unit+

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The IU asks for data from a specific address. That address is compared against
information stored in the cache tags. The S4 cache chip decides if the data the
IU is looking for is stored in the cache, based on the information stored in the
cache tags.
If the data is in the cache, the cache data RAM transmits data to the IU as fast
as the IU can receive data. If the data is not in the cache, this is referred to as
a cache miss, and the IU is halted. An SBus cycle is initiated to obtain the
required data from main memory. The cache data RAM is filled with the
information obtained from main memory and the IU is started again.
The cache design implemented in the SPARCstation 2 is a 64 K write-through
cache with one level of write buffering. The cache line size is 16 bytes, with
one tag for each line.
There are three SBus slots on the main logic board (see Figures 1-2 and 1-3).
SBus boards such as frame buffer board, a GX graphics accelerator board, or a
second ethernet board are installed in the SBus slots.
The SBus slots are connected to the SBus data bus and the SBus address bus.
The SBus is a proprietary 32-bit synchronous bus. See "SBus Boards" later in
this chapter for more information on specific SBus boards. "SBus Boards" in
Chapter 4 explains how to remove and replace SBus boards.
The S4 memory management unit+ (MMU+) chip (see Figures 1-3 and 1-4) is
connected to the system bus. The MMU+ maps the virtual addresses used by
user programs, SunOS, and input/output devices to physical memory
addresses. This is how virtual memory is implemented. Virtual memory
allows a user program to have access to an address space that is larger than the
physical memory present on the system.
In addition, it isolates the address space of one process from that of another,
preventing errors in a user-level program from bringing the entire system
down. It also controls the protections (read-only or read/write) associated
with each page of memory, allowing, for example, one copy of a shared library
to be used by many running programs.
SPARCstation 2 Field Service Manual—February 1991