TMS320LBC57PGE80 Datasheet, PDF (20/94 Pages) Texas Instruments – DIGITAL SIGNAL PROCESSORS

TMS320C5x, TMS320LC5x
DIGITAL SIGNAL PROCESSORS
SPRS030A – APRIL 1995 – REVISED APRIL 1996
32-bit ALU/accumulator
The 32-bit ALU and accumulator implement a wide range of arithmetic and logical functions, the majority of
which execute in a single cycle. The ALU is a general-purpose arithmetic/logic unit that operates on 16-bit words
taken from data memory or derived from immediate instructions. In addition to the usual arithmetic instructions,
the ALU can perform Boolean operations, facilitating the bit manipulation ability required of a high-speed
controller. One input to the ALU always is supplied by the accumulator, and the other input can be furnished
from the product register (PREG) of the multiplier, the accumulator buffer (ACCB), or the output of the scaling
shifter [which has been read from data memory or from the accumulator (ACC)]. After the ALU performs the
arithmetic or logical operation, the result is stored in the ACC where additional operations, such as shifting, can
be performed. Data input to the ALU can be scaled by the scaling shifter. The 32-bit ACC is split into two 16-bit
segments for storage in data memory. Shifters at the output of the ACC provide a left shift of 0 to 7 places. This
shift is performed while the data is being transferred to the data bus for storage. The contents of the ACC remain
unchanged. When the postscaling shifter is used on the high word of the ACC (bits 31 – 16), the most significant
bits (MSBs) are lost and the least significant bits (LSBs) are filled with bits shifted in from the low word (bits
15 – 0). When the postscaling shifter is used on the low word, the LSBs are filled with zeros.
The ’C5x supports floating-point operations for applications requiring a large dynamic range. By performing left
shifts, the normalization instruction (NORM) is used to normalize fixed-point numbers contained in the ACC.
The four bits of the TREG1 define a variable shift through the scaling shifter for the ADDT/LACT/SUBT
instructions (add to / load to / subtract from ACC with shift specified by TREG1). These instructions are useful
in denormalizing a number (converting from floating point to fixed point). They are also useful for executing an
automatic gain control (AGC) going into a filter.
The single-cycle 1-bit to 16-bit right shift of the ACC efficiently aligns the ACC’s contents. This, coupled with
the 32-bit temporary buffer on the ACC, enhances the effectiveness of the ALU in extended-precision arithmetic.
The ACCB provides a temporary storage place for a fast save of the ACC. The ACCB also can be used as an
input to the ALU. The minimum or maximum value in a string of numbers is found by comparing the contents
of the ACCB with the contents of the ACC. The minimum or maximum value is placed in both registers, and,
if the condition is met, the carry bit (C) is set to 1. The minimum and maximum functions are executed by the
CRLT and CRGT instructions, respectively.
scaling shifters
The ’C5x provides a scaling shifter that has a 16-bit input connected to the data bus and a 32-bit output
connected to the ALU. This scaling shifter produces a left shift of 0 to 16 bits on the input data. The shift count
is specified by a constant embedded in the instruction word or by the value in TREG1. The LSBs of the output
are filled with zeros; the MSBs may be either filled with zeros or sign extended, depending upon the value of
the sign-extension mode (SXM) bit of status register ST1.
The ’C5x also contains several other shifters that allow it to perform numerical scaling, bit extraction,
extended-precision arithmetic, and overflow prevention. These shifters are connected to the output of the
product register and the ACC.
parallel logic unit
The parallel logic unit (PLU) is a second logic unit, additional to the main ALU, that executes logic operations
on data without affecting the contents of the ACC. The PLU provides the bit-manipulation ability required of a
high-speed controller and simplifies control / status register operations. The PLU provides a direct logic
operation path to data memory space and can set, clear, test, or toggle multiple bits directly in a data memory
location, a control / status register, or any register that is mapped into data memory space.
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