How to store a value

We need a few more components:

• mux select one of two values

• flip-flop hold a value until told otherwise

register

Registers give us SRAM which can hold a value as long as the system has power

another way to physically store a value¶

DRAM uses one transistor and one capacitor. (SRAM uses 4-6 transistors)

a capacitor holds a charge for a time, but gradually fades, so it has to be refreshed

ROM is a diode matrix traditionally

• hardward encodes the instructions

• cannot be changed without rewiring

PROM¶

• can be reprogrammed once after device is made

• used for firmware/microcode

(programmable ROM)

EPROM or EEPROM¶

• treated like ROM

• can be erased & reprogrammed

• but not infinitely, very limited number of tmes

• holds value when power goes away, UV light or electriciy can re-program

SRAM:¶

• is volatile; it needs power to hold a value

• is fast

• takes up more space on the chip (uses 4-6 transistors per bit)

• typically used of cache and internal registers

DRAM:¶

• is volatile; it needs power to hold a value

• is slower and requires a refresh

• is small (1 transistor + 1 capacitor per bit) and space efficient

• used for the main RAM

Flash memory¶

• non-volatile; holds a value without power, can be electircally erased and re-programmed

• comprised of memory “cells” in layerd boards

• memory cells are mosfets - a different type of transistor that retains a state after power is removed and put bag

• implemented with EEPROM

• designed for large blocks and limited writes (~10k)

• used for ssd, usb flash drives, smartphones, etc

• slow to write, fast to read

Why does this matter?¶

import timeit
from numpy import round

one_line = '5+7+9'
multi_line = """\
a = 5
b = 7 
c = 9
a+b+c 
"""

one_line_time = timeit.timeit(one_line,number=10000)
multi_line_time = timeit.timeit(multi_line,number=10000)
one_line_time, multi_line_time

one_line_store = 's=5+7+9'
one_line_store_time = timeit.timeit(one_line_store,number=10000)
one_line_store_time

Prepare for Next Class¶

since this is posted late, you will get time for the following at the start of class

1. read the notes from 11/13 there is a more detailed solution to the problems we did in class

2. Trace through the ripple carry adder to see how different calculations take different amounts of time. Find up with 2 example addition problems that demonstrate how much the time can vary(one short, one long).

hint use electron mode instead of immediate and it will count. There is also a “reload” button that will reset the counter.

Badges¶

Experience Report Evidence¶

Questions After Today’s Class¶