9/28/04

```     Dear Professor Patt,

```

Would I be mean if I asked him if he looked in the index in the textbook under "address space" and "addressability" before I answered his question? I tried real hard to explain it carefully there. Or, should I just tell him?

I guess I will tell him: none.

It is hard for me to not answer a student's question and it does allow me to add some more information to the rest of you. But I also need to get everybody in the habit of first looking in the textbook and talking to the others in your study group. Success in engineering school is going to be enhanced greatly if you develop this habit early.

Moving on...

`     How do you determine the addressability knowing the address space? `

You can't. They are two COMPLETELY INDEPENDENT attributes. "Address space" tells you how many distinct locations there are; "Addressability" tells you how many bits are stored in each location. If you know one, you know NOTHING about the other.

`     For example the addressability of the LC-3 is 16.`

That is correct.

`     Was there a choice to make it anywhere between a minimum and a maximum?`

There is a choice for both addressability and for address space. The computer architect who specifies a particular ISA chooses both based on what he/she thinks makes sense for the set of applications that are expected to be carried out on computers that implement that ISA.

`     If so, what are the minimum and maximum, and how do I dertermine them?`

The minimum in each case is 1. In the case of addressability, that means each bit has its own address. In the case of address space, that means there is only one location. There are computers that have an addressability of 1. A computer with an address space of 1 would be pretty silly.

There are no real maximums. The computer architect can make the number as big as makes sense. There are computers available today with addressability of 64 bits and there are computers today with 2^64 addresses.

`     Thank you.`

You're welcome,
Yale Patt

```     Respectfully,

<< name withheld to protect the polite one who needs to visit his textbook more  ...I think >>

```