The hardware - The Computing Universe: A Journey through a Revolution - page 33

magnetic tapes are fundamentally sequential in the way they store and access
data on the tape. Accessing some data stored halfway along the tape requires
running through all the other data on the tape until that point is reached. This
is fine if we only want to read long streams of information, but it is not very
efficient if we want to access small amounts of nonsequentially stored bits of
information. For this reason, disks and solid state semiconductor memory tech-
nologies that allow “random access” directly to any piece of data stored on the
device are usually preferred, with the use of magnetic tapes restricted to pro-
viding archival backup storage for large data sets.
The fetch-execute cycle
We have now seen that computer hardware consists of many different
components, which can all be implemented in a variety of ways. How do we
coordinate and orchestrate the work of all these devices? The word orchestrate
is an appropriate analogy. In an orchestra, there is a conductor who makes sure
that everybody plays at the right time and in the right order. In the computer,
an electronic clock performs the function of the conductor. Without the pres-
ence of a clock signal, memory circuits will not operate reliably. The clock sig-
nal also determines when the logic gates are allowed to switch.
In his draft report on the EDVAC, another key idea that von Neumann intro-
duced was the “fetch-execute cycle.” This relies on an electronic clock to generate
a heartbeat that drives the computer through its series of operations. For simpli-
city and reliability, von Neumann chose the simplest possible control cycle to be
coordinated by the central control unit:
- Fetch the next instruction from memory and bring it to the control unit;
- Execute the instruction using data retrieved from memory or already
present;
- Send results back for storage in memory; and
- Repeat the fetch-execute cycle.
Von Neumann chose the approach of just getting one instruction at a time
because he feared that any other approach would make the computer too hard
to build and program reliably. Alan Perlis, one of the early programming pio-
neers, once said, “sometimes I think the only universal in the computing field
is the fetch-execute cycle.” 8
The processor or CPU is the place where the instructions are executed and
the data are manipulated. The main functions of the processor are to fetch the
instructions from the main memory, decode the instructions, fetch the data on
which the instruction's mathematical or logical operation will be performed,
execute the instructions, and store the results. These main functions have not
changed since the early processor designs. On a logical level, a simple processor
(see Fig. 2.21 ) consists of a bank of registers, an arithmetical logical unit (ALU),
and control unit (CU).
The CU fetches instructions from the memory, decodes them and generates
the sequence of control signals that are required for completing the instruc-
tions. The ALU performs the arithmetical and logical operations. For an execu-
tion of each instruction, various components need to be connected by switches