Results (
Thai) 2:
[Copy]Copied!
We have seen that digital systems consist of many components, including arithmetic
logic units, registers, memory, decoders, and control units. These units are
interconnected by buses to allow information to flow through the system. The
instruction set presented for MARIE in the preceding sections constitutes a set of
machine level instructions used by these components to execute a program. Each
instruction appears to be very simplistic; however, if you examine what actually
happens at the component level, each instruction involves multiple operations.
For example, the Load instruction loads the contents of the given memory location
into the AC register. But, if we observe what is happening at the component
level, we see that multiple “mini-instructions” are being executed. First, the
address from the instruction must be loaded into the MAR. Then the data in
memory at this location must be loaded into the MBR. Then the MBR must be
loaded into the AC. These mini-instructions are called microoperations and specify
the elementary operations that can be performed on data stored in registers.
The symbolic notation used to describe the behavior of microoperations is
called register transfer notation (RTN) or register transfer language (RTL). We
use the notation M[X] to indicate the actual data stored at location X in memory,
and ← to indicate a transfer of information. In reality, a transfer from one register
to another always involves a transfer onto the bus from the source register, and
then a transfer off the bus into the destination register. However, for the sake of
clarity, we do not include these bus transfers, assuming that you understand that
the bus must be used for data transfer.
We now present the register transfer notation for each of the instructions in
the ISA for MARIE.
Load X
Recall that this instruction loads the contents of memory location X into the AC.
However, the address X must first be placed into the MAR. Then the data at location
M[MAR] (or address X) is moved into the MBR. Finally, this data is placed
in the AC.
Being translated, please wait..
