Memory

The kind of answer

Dated. A flip-flop gives out now what it was given one tick ago. It is the first part of the machine with a past, and the clock is what dates it. The unit’s rule holds here too: first the order of the ticks, then the values.

On paper first

Draw a clock as a row of ticks along the bottom of a page. Above it, write what a one-bit memory holds at each tick if you tell it to “load 1” at tick 3 and “load 0” at tick 7.

What you build

In the Hardware Simulator, choose Project 3 from the list at the top, then each chip in turn.

• Bit
• Register
• RAM8
• RAM64
• RAM512
• RAM4K
• RAM16K
• PC

Each one is done when its test passes in the IDE. A pass cannot be argued with, and neither can a fail.

Week by week

About an afternoon a week. If a week runs long, let it: the strand keeps its own order, not the calendar’s.

Week	Watch	Do
Week 13	3.1; 3.2	Ticks and the flip-flop. Build Bit, then Register out of Bits.
Week 14	3.3	RAM8, then RAM64 out of RAM8s: the same pattern again, one size up.
Week 15	3.4; 3.5; 3.6	RAM512, RAM4K and RAM16K, then the program counter, PC.

The lectures

6 lectures, about 84 minutes in all, by Nisan and Schocken. Watch with your parent, one or two at a sitting.

• 3.1 Sequential Logic 9:46
• 3.2 Flip Flops 15:43
• 3.3 Memory Units 25:41
• 3.4 Counters 10:17
• 3.5 Project 3 Overview 10:54
• 3.6 Perspectives 11:40

For the parent

The flip-flop (DFF) is the one chip the course gives rather than has you build. It is taken on trust, as a given. A wire may loop back on itself only through a DFF; that is how the clock gives the machine a past.