IDT79R4700
of mappings can be locked into the TLB and avoid being randomly
replaced. This facilitates the design of real-time systems, by allowing
deterministic access to critical software.
The RC4700 processor also supports a supervisor mode in which the
virtual address space is 256.5GB (2.5GB in 32-bit address mode),
divided into three regions that are based on the high-order bits of the
virtual address. If the RC4700 is configured for 64-bit virtual addressing,
the virtual address space layout is an upwardly compatible extension of
the 32-bit virtual address space layout. Figure 4 on page 5 shows the
address space layout for the 32-bit virtual address operation.
The joint TLB also contains information to control the cache coher-
ency protocol for each page. Specifically, each page has attribute bits to
determine whether the coherency algorithm is uncached, non-coherent
write-back, non-coherent write-through write-allocate or non-coherent
write-through no write-allocate. Non-coherent write-back is typically
used for both code and data on the RC4700; however, hardware-based
cache coherency is not supported.
Memory Management Unit (MMU)
The Memory management unit controls the virtual memory system
page mapping. It consists of an instruction address translation buffer
(the ITLB), a data address translation buffer (the DTLB), a Joint TLB (the
JTLB), and co-processor registers used for the virtual memory mapping
sub-system.
0xFFFFFFFF
Kernel virtual address space
(kseg3)
0xE0000000
0xDFFFFFFF
Mapped, 0.5GB
Supervisor virtual address space
(sseg)
Instruction TLB (ITLB)
Mapped, 0.5GB
The RC4700 also incorporates a two-entry instruction TLB. Each
entry maps a 4KB page. The instruction TLB improves performance by
allowing instruction address translation to occur in parallel with data
address translation. When a miss occurs on an instruction address
translation, the least-recently used ITLB entry is filled from the JTLB.
The operation of the ITLB is invisible to the user.
0xC0000000
0xBFFFFFFF
Uncached kernel physical address space
(kseg1)
0xA0000000
0x9FFFFFFF
Unmapped, 0.5GB
Cached kernel physical address space
(kseg0)
Data TLB (DTLB)
Unmapped, 0.5GB
The RC4700 also incorporates a four-entry data TLB. Each entry
maps a 4KB page. The data TLB improves performance by allowing
data address translation to occur in parallel with instruction address
translation. When a miss occurs on a data address translation, the DTLB
is filled from the JTLB. The DTLB refill is pseudo-LRU: the least recently
used entry of the least recently used half is filled. The operation of the
DTLB is invisible to the user.
0x80000000
0x7FFFFFF
User virtual address space
(useg)
Mapped, 2.0GB
Joint TLB (JTLB)
For fast virtual-to-physical address decoding, the RC4700 uses a
large, fully associative TLB that maps 96 virtual pages to their corre-
sponding physical addresses. The TLB is organized as 48 pairs of even-
odd entries and maps a virtual address and address space identifier into
the large, 64GB physical address space.
0x00000000
Figure 4 Kernel Mode Virtual Addressing (32-bit Mode)
Cache Memory
Two mechanisms are provided to assist in controlling the amount of
mapped space and the replacement characteristics of various memory
regions. First, the page size can be configured, on a per-entry basis, to
map a page size of 4KB to 16MB (in multiples of 4). A CP0 register is
loaded with the page size of a mapping, and that size is entered into the
TLB when a new entry is written. Thus, operating systems can provide
special purpose maps; for example, a typical frame buffer can be
memory mapped using only one TLB entry.
To keep the RC4700’s high-performance pipeline full and operating
efficiently, the RC4700 incorporates on-chip instruction and data caches
that can be accessed in a single processor cycle. Each cache has its
own 64-bit data path and can be accessed in parallel.
Instruction Cache
The RC4700 incorporates a two-way set associative on-chip instruc-
tion cache. This virtually indexed, physically tagged cache is 16KB in
size and is protected with word parity.
The second mechanism controls the replacement algorithm, when a
TLB miss occurs. The RC4700 provides a random replacement algo-
rithm to select a TLB entry to be written with a new mapping; however,
the processor provides a mechanism whereby a system specific number
5 of 25
April 10, 2001
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