Hardware Reference
In-Depth Information
where
m
is the number of memory locations in the memory. The set of conductor wires that
carry address signals is referred to as the
address bus
.
The number of conductor wires that carry the data must be equal to the number of bits in
each memory location. The set of conductors that carry the data to be accessed is called the
data bus
.
To read a memory location, the processor sends out the address of the memory location to
be accessed and applies a logic 1 (high voltage) to the RD signal and a logic 0 to the WR signal
(this specifies a read operation). In response, the memory system decodes the address input and
enables the specified memory location to send out its contents to the data bus to be read by the
processor.
To write a value to a location of the memory system in Figure 1.2, the processor places the
data to be written on the data bus and places the address of the memory location on the address
bus and applies a logic 1 (high voltage) to the WR signal and a logic 0 to the RD signal (this
specifies a write operation). In response, the memory system uses its address decoder to select a
location and writes the value on the data bus to that location.
The actual memory system design and the signals involved may be different from those in
Figure 1.2 but the concept would be the same. The semiconductor vendors may use RD instead
of RD and W
R instead of WR to refer to read and write signals. These types of signals are active
low; that is, when they are low, they are considered to be at logic 1.
In order to allow the computer to execute the program immediately after the power is
turned on, part of the program must be stored in nonvolatile memory. Some computers placed
the startup program in the nonvolatile memory, which will perform the system initialization.
After the system initialization is completed, it loads additional programs from secondary stor-
age such as hard disk or optical storage into the semiconductor memory (often called
main
memory
) for execution. Mainframe computers, workstations, and personal computers follow
this approach. After power is turned on, the processor starts to execute the program from the
BIOS, which performs the system initialization. After system initialization is completed, the
processor loads additional programs such as Windows operating system into the main memory
for execution. Other computers, including most embedded systems, place all their programs in
the nonvolatile memory. After power-up, the processor starts to execute the program from the
nonvolatile memory.
The following sections deal with several important issues related to program execution.
The program counter consists of flip-flops and other additional logic gates. There are sev-
eral types of flip-flops in use. Among them, the D-type flip-flop is the most popular one. The
circuit of a D-type flip-flop with set and reset capability is shown in Figure 1.3.
In Figure 1.3,
•
Depending on the design, the D value may be transferred to Q on either the rising
or the falling edge (but not both edges) of the CLK input.
•
The CLK signal is the clock input signal of the D flip-flop.
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