What is "expensive" vs "inexpensive" memory?

Question

I've been reading that accessing data from lower components in the memory hierarchy is slower but less expensive. For example, fetching data from registers is fast but expensive. Could someone explain what 'expensive' means here? Is it literally the dollar cost of components? If so, I don't understand why faster components would be more expensive. I read this answer (Memory Hierarchy - Why are registers expensive?) and it talks about how accessing data in registers requires additional data paths that aren't required by lower memory components, but I didn't understand from any of the examples why those data paths would be required when fetching from registers, but not from something like main memory.

So to summarize, my two questions are:

1) What does 'expensive' mean in this context?

2) Why are faster areas of memory like registers more expensive?

Thanks!

Answer 1

1) What does 'expensive' mean in this context?

Expensive has the usual meaning ($). For integrated circuit, price depends on circuit size and is directly related to the number and size of transistors. It happens that and "expensive" memory requires more area on an integrated circuit.

Actually, there are several technologies used to implement a memorizing device.

Registers that are used in processor. They are realized with some kind of logic devices called latches, and their main quality is to be fast, in order to allow two reads/one write per cycle. To that purpose, transistors are dimensioned to improve driving. It depends on the actual design, but typically a bit of memory requires ~10 transistors in a register.

Static memory (SRAM) is designed as a matrix of simplified flip-flops, with 2 inverters per cell and only requires 6 transistors per memorized bit. More, static memory is a memory and to improve the number of bits per unit area, transistors are designed to be smaller than for registers. SRAM are used in cache memory.

Dynamic memory (DRAM) uses only a unique transistor as a capacitance for memorization. The transistor is either charged or discharged to represent a 1 or 0. Though extremely economic, this technique cannot be very fast, especially when a large number of cells is concerned as in present DRAM chips. To improve capacity (number of bits on a given area), transistors are rendered as small as possible, and a complex analog circuitry is used to detect small voltage variations to speed up cell content reading. More reads destroys the cell content and requires a write. Last, there are leaks in the capacitance and data must be periodically rewritten to insure data integrity. Put altogether, it makes a DRAM a slow device, with an access time of 100-200 processor cycles, but they can provide extremely cheap physical memory.

2) Why are faster areas of memory like registers more expensive?

Processor rely on a memory hierarchy and different level of the hierarchy have specific constraints. To make a cheap memory, you need small transistors to reduce the size required to memorize a bit. But for electrical reasons, a small transistor is a poor generator that cannot provide enough current to drive rapidly its output. So, while the underlying technology is similar, design choices are different in different part of the memory hierarchy.

Answer 2

What does 'expensive' mean in this context?

More and larger transistors (more silicon per bit), more power to runs those transistors

Why are faster areas of memory like registers more expensive?

All memory is made as cheap as it can be for the needed speed -- there's no reason to make it more expensive if it doesn't need to be, or slower if it doesn't need to be. So it becomes a tradeoff and finding "sweet spots" in the design space -- make a particular type of circuit as fast and cheap as possible, while a different circuit is also tuned to be as fast and cheap as possible. If one design is both slower and more expensive, then there's no reason to ever use it. Its only when one design is faster while the other is cheaper, does it make sense to use both in different parts of the system.