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IDT79R4640 Datasheet(PDF) 3 Page - Integrated Device Technology

Part No. IDT79R4640
Description  Low-Cost Embedded 64-bit RISController w/ DSP Capability
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Maker  IDT [Integrated Device Technology]
Homepage  http://www.idt.com
Logo IDT - Integrated Device Technology

IDT79R4640 Datasheet(HTML) 3 Page - Integrated Device Technology

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April 10, 2001
The MIPS-III architecture defines that the results of a multiply or
divide operation are placed in the HI and LO registers. The values can
then be transferred to the general purpose register file using the MFHI/
MFLO instructions.
The RC4640 adds a new multiply instruction, “MUL”, which can
specify that the multiply results bypass the “Lo” register and are placed
immediately in the primary register file. By avoiding the explicit “Move-
from-Lo” instruction required when using “Lo”, throughput of multiply-
intensive operations is increased.
An additional enhancement offered by the RC4640 is an atomic
“multiply-add” operation, MAD, used to perform multiply-accumulate
operations. This instruction multiplies two numbers and adds the product
to the current contents of the HI and LO registers. This operation is used
in numerous DSP algorithms, and allows the RC4640 to cost reduce
systems requiring a mix of DSP and control functions.
Finally, aggressive implementation techniques feature low latency for
these operations along with pipelining to allow new operations to be
issued before a previous one has fully completed. Table 1 also shows
the repeat rate (peak issue rate), latency, and number of processor stalls
required for the various operations. The RC4640 performs automatic
operand size detection to determine the size of the operand, and imple-
ments hardware interlocks to prevent overrun, allowing this high-perfor-
mance to be achieved with simple programming.
Floating-Point Coprocessor
Floating-Point Coprocessor
Floating-Point Coprocessor
Floating-Point Coprocessor
The RC4640 incorporates an entire single-precision floating-point
coprocessor on chip, including a floating-point register file and execution
units. The floating-point coprocessor forms a “seamless” interface with
the integer unit, decoding and executing instructions in parallel with the
integer unit.
The floating-point unit of the RC4640 directly implements single-
precision floating-point operations, which enables the RC4640 to
perform functions such as graphics rendering without requiring exten-
sive die area or power consumption. The single-precision unit of the
RC4640 is directly compatible with the single-precision operation of the
RC4700, and features the same latencies and repeat rates.
The RC4640 does not directly implement the double-precision opera-
tions found in the RC4700. However, to maintain software compatibility,
the RC4640 will signal a trap when a double-precision operation is initi-
ated, allowing the requested function to be emulated in software. Alter-
natively, the system architect could use a software library emulation of
double-precision functions, selected at compile time, to eliminate the
overhead associated with trap and emulation.
Floating-Point Units
Floating-Point Units
Floating-Point Units
Floating-Point Units
The RC4640’s floating-point execution units perform single precision
arithmetic, as specified in IEEE Standard 754. The execution unit is
broken into a separate multiply unit and a combined add/convert/divide/
square root unit. Overlap of multiply and add/subtract is supported. The
multiplier is partially pipelined, allowing a new multiplication instruction
to begin every 6 cycles.
As in the IDT79RC4700, the RC4640 maintains fully precise floating-
point exceptions while allowing both overlapped and pipelined opera-
tions. Precise exceptions are extremely important in mission-critical
environments, such as ADA, and highly desirable for debugging in any
The floating-point unit’s operation set includes floating-point add,
subtract, multiply, divide, square root, conversion between fixed-point
and floating-point format, conversion among floating-point formats, and
floating-point compare. These operations comply with IEEE Standard
754. Double precision operations are not directly supported; attempts to
execute double-precision floating point operations, or refer directly to
double-precision registers, result in the RC4640 signalling a “trap” to the
CPU, enabling emulation of the requested function. Table 2 gives the
latencies of some of the floating-point instructions in internal processor
Floating-Point General Register File
Floating-Point General Register File
Floating-Point General Register File
Floating-Point General Register File
The floating-point register file is made up of thirty-two 32-bit regis-
ters. These registers are used as source or target registers for the
single-precision operations.
References to these registers as 64-bit registers (as supported in the
RC4700) will cause a trap to be signalled to the integer unit.
The floating-point control register space contains two registers; one
for determining configuration and revision information for the copro-
cessor and one for control and status information. These are primarily
involved with diagnostic software, exception handling, state saving and
restoring, and control of rounding modes.
Table 2 Floating-Point Operation

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