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Chapter 1 Architectural Overview
Special Purpose
Reg. No.
Figure 1-21.
Additional Special Purpose Register for the Am29050 Microprocessor
Mnemonic
Unprotected Registers
Exception Opcode
164
EXOP
data busses together (disabling the Harvard architecture advantages by creating a
2–bus system) or providing an off–chip bridge. This bridge must enable the address
space to be reached from within some range of data memory space, at least for word–
size read accesses, and, all be it, with additional access time penalties.
The Am29050 processor has an additional group of registers known as the float-
ing–point accumulators. There are four 64–bit accumulators ACC3–0 which can be
used with certain floating–point operations. They can hold double– or single–preci-
sion numbers. They are not special registers in the sense they lie in special register
space. They are located in their own register space, giving the Am29050 one more
register space than the normal three register spaces of the other 29K family members.
However, like special registers, they can only be accessed by move–to and move–
from accumulator type instructions.
Double–precision numbers (64–bit) can be moved between accumulators and
general registers in a single cycle. Global registers are used in pairs for this operation.
This is possible because the Am29050 processor is equipped with an additional
64–bit write–back port for floating point data, and the register file is implemented
with a width of 64–bits.
1.10.3 Translation Look–Aside Registers
Although some 29K family members are equipped with region mapping regis-
ters, a Translation Look–Aside Buffer (TLB) technique is generally used to provide
virtual to physical address translation. The TLB is two–way set associative and up to
64 translations are cached in the TLB support registers.
The TLB registers form the basis for implementing a Memory Management
Unit. The scheme for reloading TLB registers is not dictated by processor micorcode,
but left to the programmer to organize. This enables a number of performance boost-
ing schemes to be implemented with low overhead costs. However, it does place the
burden of creating a TLB maintenance scheme on the user. Those used to having to
work around a processor’s microcode imposed scheme will appreciate the freedom.
TLB registers can only be accessed by move–to TLB and move–from TLB
instructions executed by the processor operating in Supervisor mode. Each of the
possible 64 translation entries (less than 64 with some 29K family members) requires