WORD SIZE
All common floating-point DSPs use a 32-bit data word. For fixed-point DSPs, the most common data word size is 16 bits. Motorola’s DSP563xx family uses a 24-bit data word, however, while Zoran’s ZR3800x family uses a 20-bit data word. The size of the data word has a major impact on cost, because it strongly influences the size of the chip and the number of package pins required, as well as the size of external memory devices connected to the DSP. Therefore, designers try to use the chip with the smallest word size that their application can tolerate.
As with the choice between fixed and floating point chips, there is often a trade-off between word size and development complexity.
RELATION BETWEEN DATA WORD SIZE AND INSTRUCTION WORD SIZE- Dspa word size
1. RELATION BETWEEN
DATA WORD SIZE
AND
INSTRUCTION WORD SIZE
DIGITAL SIGNAL PROCESSOR
AND ARCHITECTURE
NITHIN KALLEPALLY
2. • In computing, a word is the natural unit of data used by a particular processor design.
A word is a fixed-sized piece of data handled as a unit by the instruction set or the
hardware of the processor.The number of bits in a word (the word size, word width,
or word length) is an important characteristic of any specific processor design
or computer architecture.
• The size of a word is reflected in many aspects of a computer's structure and operation;
the majority of the registers in a processor are usually word sized and the largest piece of
data that can be transferred to and from the working memory in a single operation is a
word in many (not all) architectures.The largest possible address size, used to designate
a location in memory, is typically a hardware word (here, "hardware word" means the
full-sized natural word of the processor, as opposed to any other definition used).
3. • WORD SIZE
• All common floating-point DSPs use a 32-bit data word. For fixed-point
DSPs, the most common data word size is 16 bits. Motorola’s DSP563xx
family uses a 24-bit data word, however, while Zoran’s ZR3800x family uses
a 20-bit data word.The size of the data word has a major impact on cost,
because it strongly influences the size of the chip and the number of
package pins required, as well as the size of external memory devices
connected to the DSP.Therefore, designers try to use the chip with the
smallest word size that their application can tolerate.
• As with the choice between fixed and floating point chips, there is often a
trade-off between word size and development complexity.
4. • For example, with a 16-bit fixed-point processor, a programmer can
perform double-precision 32-bit arithmetic operations by stringing together
an appropriate combination of instructions. (Of course, double-precision
arithmetic is much slower than single-precision arithmetic.) If the bulk of an
application can be handled with single-precision arithmetic, but the
application needs more precision for a small section of the code, the
selective use of double-precision arithmetic may make sense. If most of the
application requires more precision, a processor with a larger data word size
is likely to be a better choice.
5. • Note that while most DSP processors use an instruction word size
equal to their data word size, not all does.The Analog Devices ADSP-
21xx family, for example, uses a 16-bit data word and a 24-bit
instruction word.
6. •An instruction is assembled in binary form(0,1) known as
machine code or opcode. Due to different ways of specifying
data or operand the machine code are not same for all the
instruction .The size of an instruction signifies how much
memory space is required to load an instruction in the
memory.
INSTRUCTION WORD
7. • 1. One-byte instructions –
In 1-byte instruction, the opcode and the operand of an instruction are represented in one byte.
• Note –The length of these instructions is 8-bit; each requires one memory location.
• 2.Two-byte instructions –
Two-byte instruction is the type of instruction in which the first 8 bits indicates the opcode and the next 8
bits indicates the operand.
• Note –This type of instructions need two bytes to store the binary codes.
• 3.Three-byte instructions –
Three-byte instruction is the type of instruction in which the first 8 bits indicates the opcode and the next
two bytes specify the 16-bit address.The low-order address is represented in second byte and the high-
order address is represented in the third byte.
• Note –These instructions would require three memory locations to store the binary codes
8. Precision (Fixed-Point Binary Representation)
Dynamic Range (# of bits per data word x 6 db/bit or resolution)
16-bit 96 dB
24-bit 144 dB
32-bit 192 dB
When a computer architecture is designed, the choice of a word size is of substantial importance.There are
design considerations which encourage particular bit-group sizes for particular uses (e.g. for addresses), and
these considerations point to different sizes for different uses. However, considerations of economy in
design strongly push for one size, or a very few sizes related by multiples or fractions (submultiples) to a
primary size.That preferred size becomes the word size of the architecture.
