
CHAPTER 1 INTRODUCTION
User’s Manual U14272EJ3V0UM
44
1.4.5 Floating-point unit (FPU)
The V
R
4181 does not support the floating-point unit (FPU). Coprocessor Unusable exception will occur if any FPU
instructions are executed. If necessary, FPU instructions should be emulated by software in an exception handler.
1.4.6 Memory management unit
The V
R
4181 has a 32-bit physical addressing range of 4 GB. However, since it is rare for systems to implement a
physical memory space as large as that memory space, the CPU provides a logical expansion of memory space by
translating addresses composed in the large virtual address space into available physical memory addresses.
The V
R
4181 has three operating modes: User, Supervisor, and Kernel. The manner in which memory addresses
are mapped depends on these operating modes.
In addition, the V
R
4181 supports the 32-bit and 64-bit addressing modes. The manner in which memory
addresses are translated or mapped depends on these addressing modes.
A detailed description of the physical address space is given in
CHAPTER 4 MEMORY MANAGEMENT
SYSTEM
. For details about the virtual address space, refer to
V
R
4100 Series Architecture User’s Manual
.
(1) Translation lookaside buffer (TLB)
Virtual memory mapping is performed using the translation lookaside buffer (TLB). The TLB translates virtual
addresses to physical addresses. It runs by a full-associative method and has 32 entries, each of which two
successive pages are mapped.
The TLB of the V
R
4181 holds both instruction addresses and data addresses so that it is called as joint TLB
(JTLB).
The page size can be configured, on a per-entry basis, to map a page size of 1 KB to 256 KB, in power of four. A
CP0 register stores the size of the page to be mapped, 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.
Translating a virtual address to a physical address begins by comparing the virtual address from the processor
with the physical addresses in the TLB. There is a match when the virtual page number (VPN) of the address is
the same as the VPN field of an entry, and either the Global (G) bit of the TLB entry is set, or the ASID field of the
virtual address is the same as the ASID field of the TLB entry.
This match is referred to as a TLB hit. If there is no match, a TLB Miss exception is taken by the processor and
software is allowed to refill the TLB from a page table of virtual/physical addresses in memory.
1.4.7 Cache
The V
R
4181 chip incorporates instruction and data caches, which are independent of each other. This
configuration enables high-performance pipeline operations. Both caches have a 64-bit data bus, enabling a one-
clock access. These buses can be accessed in parallel. The instruction cache of the V
R
4181 has a storage capacity
of 4 KB, while the data cache has a capacity of 4 KB.
For details about caches, refer to
V
R
4100 Series Architecture User’s Manual
.
1.4.8 Instruction pipeline
The V
R
4181 has a 5-stage instruction pipeline. Under normal circumstances, one instruction is issued each cycle.
For details, refer to
V
R
4100 Series Architecture User’s Manual
.