Page 130 - A Practical Guide from Design Planning to Manufacturing

Computer Architecture 103

TABLE 4-7 Relative Instruction Count
Architectural type Instruction count
Pure register 198%
Register/memory 178%
Pure memory 114%

architecture, which can provide more compact code. The RISC archi-
tectures are three-operand architectures. The VAX architecture, seek-
ing the greatest possible flexibility in instruction type, provides for both
two- and three-operand formats.
The number and type of operands supported by different instructions
will have a great effect on how these instructions can be encoded.
Allowing for different operand encoding can greatly increase the func-
tionality and complexity of a computer architecture. The resulting size
of code and complexity in decoding will have an impact on performance.

Data transfer instructions
In addition to computational instructions, any computer architecture
will have to include data transfer instructions for moving data from
one location to another. Values may be copied from main memory to the
processor or results written out to memory. Most architectures define
registers to hold temporary values rather than requiring all data to be
accessed by a memory address. Some common data transfer instructions
and their mnemonics are listed in Table 4-8.
Loads and stores move data to and from registers and main memory.
Moves transfer data from one register to another. The conditional move
only transfers data if some specific condition is met. This condition
might be that the result of a computation was 0 or not 0, positive or not
positive, or many others. It is up to the computer architect to define all
the possible conditions that can be tested. Most architectures define a
special flag register that stores these conditions. Conditional moves can

TABLE 4-8 Data Transfer Instructions
LD Load value from memory to a register
ST Store value from a register to memory
MOV Move value from register to register
CMOV Conditionally move value from register to register if
a condition is met
PUSH Push value onto top of stack
POP Pop value from top of stack