Multiprogramação

1. Multiprogramming is the rapid switching of the CPU between multiple processes

in memory. It is commonly used to keep the CPU busy while one or

more processes are doing I/O.

2. Input spooling is the technique of reading in jobs, for example, from cards,

onto the disk, so that when the currently executing processes are finished,

there will be work waiting for the CPU. Output spooling consists of first

copying printable files to disk before printing them, rather than printing directly

as the output is generated. Input spooling on a personal computer is not

very likely, but output spooling is.

3. The prime reason for multiprogramming is to give the CPU something to do

while waiting for I/O to complete. If there is no DMA, the CPU is fully occupied

doing I/O, so there is nothing to be gained (at least in terms of CPU utilization)

by multiprogramming. No matter how much I/O a program does, the

CPU will be 100% busy. This of course assumes the major delay is the wait

while data are copied. A CPU could do other work if the I/O were slow for

other reasons (arriving on a serial line, for instance).

4. It is still alive. For example, Intel makes Pentium I, II, and III, and 4 CPUs

with a variety of different properties including speed and power consumption.

All of these machines are architecturally compatible. They differ only in

price and performance, which is the essence of the family idea.

5. A 25 × 80 character monochrome text screen requires a 2000-byte buffer. The

1024 × 768 pixel 24-bit color bitmap requires 2,359,296 bytes. In 1980 these

two options would have cost $10 and $11,520, respectively. For current

prices, check on how much RAM currently costs, probably less than $1/MB.

6. Consider fairness and real time. Fairness requires that each process be allocated

its resources in a fair way, with no process getting more than its fair

share. On the other hand, real time requires that resources be allocated based

on the times when different processes must complete their execution. A realtime

process may get a disproportionate share of the resources.