Difference between revisions of "CSC103: DT's Notes 1"
| Line 1,132: | Line 1,132: | ||
Before we figure out what kind of number ''code'' the processor can understand, let's talk for an instant about the role of the processor relative to the memory. The processor is a machine that constantly reads numbers from memory. It normally starts with the word stored in the cell with label 0 (we'll say the ''memory cell at Address 0''), reads its contents, then moves on to the next word at ''Address 1'', then the next one at ''Address 2'', and so on. In order to keep track of where to go next, it keeps the address of the cell it is going to access in a special word it keeps internally called '''Program Counter''', or '''PC''' for short. PC is a special memory word that is '''inside''' the processor. It doesn't have an address. We call such memory words when they are inside the processor '''registers'''. | Before we figure out what kind of number ''code'' the processor can understand, let's talk for an instant about the role of the processor relative to the memory. The processor is a machine that constantly reads numbers from memory. It normally starts with the word stored in the cell with label 0 (we'll say the ''memory cell at Address 0''), reads its contents, then moves on to the next word at ''Address 1'', then the next one at ''Address 2'', and so on. In order to keep track of where to go next, it keeps the address of the cell it is going to access in a special word it keeps internally called '''Program Counter''', or '''PC''' for short. PC is a special memory word that is '''inside''' the processor. It doesn't have an address. We call such memory words when they are inside the processor '''registers'''. | ||
| − | + | <br /> | |
| + | <bluebox> | ||
;Definition | ;Definition | ||
: A ''register'' is a memory word inside the processor. A processor contains only a handful of registers. | : A ''register'' is a memory word inside the processor. A processor contains only a handful of registers. | ||
| − | + | </bluebox> | |
| + | <br /> | ||
[[Image:Calculator.jpg|right|150px]] | [[Image:Calculator.jpg|right|150px]] | ||
The processor has three important registers that allow it to work in this machine-like fashion: the '''PC''', the '''Accumulator''' (shortened to '''AC'''), and the '''Instruction Register''' ('''IR''' for short). The PC is used to "point" to the address in memory of the next word to bring in. When this number enters the processor, it must be stored somewhere so that the processor can figure out what kind of action to take. This holding place is the '''IR''' register. The way the '''AC''' register works is best illustrated by the way we use a regular hand calculator. Whenever you enter a number into a calculator, it appears in the display of the calculator, indicating that the calculator actually holds this value somewhere internally. When you type a new number that you want to add to the first one, the first number disappears from the display, but you know it is kept inside because as soon as you press the = key the sum of the first and of the second number appears in the display. It means that while the calculator was displaying the second number you had typed, it still had the first number stored somewhere internally. For the processor there is a similar register used to keep intermediate results. That's the '''AC''' register. | The processor has three important registers that allow it to work in this machine-like fashion: the '''PC''', the '''Accumulator''' (shortened to '''AC'''), and the '''Instruction Register''' ('''IR''' for short). The PC is used to "point" to the address in memory of the next word to bring in. When this number enters the processor, it must be stored somewhere so that the processor can figure out what kind of action to take. This holding place is the '''IR''' register. The way the '''AC''' register works is best illustrated by the way we use a regular hand calculator. Whenever you enter a number into a calculator, it appears in the display of the calculator, indicating that the calculator actually holds this value somewhere internally. When you type a new number that you want to add to the first one, the first number disappears from the display, but you know it is kept inside because as soon as you press the = key the sum of the first and of the second number appears in the display. It means that while the calculator was displaying the second number you had typed, it still had the first number stored somewhere internally. For the processor there is a similar register used to keep intermediate results. That's the '''AC''' register. | ||
Revision as of 18:55, 28 September 2013
--© D. Thiebaut 08:10, 30 January 2012 (EST)
