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cpe229_lec15 - CPE 229 Course Notes Lecture 15 Copyright...

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CPE 229 Course Notes: Lecture 15 Copyright: 2005 Bryan Mealy A Few Thoughts In the previous set of notes, we presented a design for a relatively simple computer. The approach we took was to first design each of the functional blocks of the computer and then we assembled these functional blocks into what was forever known as the simple computer. Although we called it a simple computer, it really was not complete because we neglected to give our simple computer design some way to input and output data to and from the outside world. We neglected this in order to somewhat reduce the scope of our discussion of computer design in general. Using what you already know about computers and digital design in general, it is a simple matter to add the instructions and support hardware to implement some simple form of I/O. But time is short and we’ll not go there now; we will, however, revisit these topics during the final week of the course. The last set of notes was focused primarily on the high-level details of the simple computer. This included the basic structure of the simple computer as well as a general description of how the simple computer executed several instructions of an arbitrary program. This high-level view of the computer somewhat necessitated the need to ignore some of the low-level details. In this set of notes, we’ll go back and take a look at some of those low-level details. These low-level details are focused on the control signals of the various functional blocks of the simple computer. The approach we’ll take in this set of notes is to take a closer look at these signals and generate timing diagrams that are used to control the underlying hardware. From there, we’ll implement the Control Unit using VHDL behavioral modeling (the technique we discussed earlier in the quarter). But before we do all that, let’s do a quick review of what we discussed in the last set of notes as well as some topics that were not discussed. A Review and Overview The previous set of notes highlighted the fact that computer was nothing more than a piece of hardware controlled by a set of instructions which were stored in memory. When controlled properly, the hardware could generate a result by crunching through some numbers. The crunching of the number happened in the ALU under control of the control unit which was in turn under direction of the instructions. Although it was not mentioned in the previous set of notes, the computer was crunching data that was somehow read in from the outside world; the result of this number crunching was then mostly likely output back into the outside world (in the form of a meaningful result). The heart of the simple computer was the ALU shown in Figure 1(a). Most of the highlights of this devices are listed below. A black box for the device is shown in Figure 1(b) which highlights the ALU’s seven control inputs and two status outputs. The data width of our computer was considered to be eight bits.
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