ITEC 352 Lecture 24 Memory

Review Questions? Reminder: HW due on Wed. Night Intel 8080 CPU

Memory Outline Memory –How it works

Memory When the copy of your floppy's getting sloppy on the disk, and the microcode instructions cause unnecessary RISC, then you have to flash your memory and you'll want to RAM your ROM. quickly turn off your computer and be sure to tell your mom! … “ - Gene Ziegler (Cornell University) Fun quote

Memory The Memory Hierarchy: What is faster?

Memory RAM What we in layman terms usually call as “main memory” It is a collection of registers. Hence, basic building block is a flip flop. What is random about RAM? “Any memory location in the RAM can be accessed in the same amount of time”

Memory Behavior Activate the chip. Select can be wired to a clock and/or other inputs (e.g., enable) This is a bi- directional line to both read and write

Memory SRAM / DRAM SRAM (Static RAM): Based on flip flops. They are considered static because the contents of each flip flop persist as long as power is applied to the chips. DRAM (Dynamic RAM): employ a capacitor which stores a minute amount of electric charge (representing 1 or 0). –Capacitors tend to be smaller than flip flops so DRAMs can be used to develop lot of memory in smaller sizes than SRAMs. –DRAMs must be refreshed often to maintain charge. –Can you recognize any of the following names: – SDRAM, DDR – RAM, DDR2 RAM? Where do they fit in: SRAM or DRAM © Wikipedia

Memory RAM Diagram Address lines. There are “m” bits in the address, hence we can address: 2 m words. HOW MANY BITS can we store in this RAM chip. Assume, word = w bits When WR = 0, write into the chip. Else read from the chip. CS:Chip Select When CS = 0, activate (or select the chip). Data lines (connected to data bus). The line can read/write one word (hence w bits) at a time. Notice: it is bi-directional

Memory Deeper A RAM chip is simply a collection of registers. Which are simply a collection of D flip flops. RAM chip: collection of registers Registers: Collection of flip flops D flip flops. If there are n flip flops, then each register is an n bit register.

Memory Deeper(2) Goal: Develop a RAM that can store 4 four bit words. How many registers must the RAM have? How many bits per register? How many bits should we have in the Address line? How many bits should we have for the data line?

Memory Deeper(2) How many registers must the RAM have? Answer: 4 registers to store 4 words. How many bits per register? Answer: 4 bits, because each word is 4 bits. (Also, this means we need 4 flip flops) How many bits should we have in the Address line? Answer: 2 bits. Because we need to be able to select one of the 4 registers. How many bits should we have for the data line? Answer: 4 bits. The data is going to be a word

Memory Deeper look into RAM (3) Architectural details: RAM requires 4 registers. Let’s represent them by boxes. What about, the addressing the registers: we need a decoder to determine which register to pick.

Memory A Four-Word Memory with Four Bits per Word in a 2D Organization

Memory Simplified Graphic

Memory Decoder usage Consider the decoder we just used. Its circuit diagram looks like this: (1)How many AND gates are we using here? (2)How many inputs do we have for each AND gate? How many AND gates would we need for a Decoder in a 64 Mb RAM?

Memory So far, so good. But can we really use a decoder? Consider the decoder we just used. Its circuit diagram looks like this: (1)How many AND gates are we using here? (2)How many inputs do we have for each AND gate? How many AND gates would we need for a Decoder in a 64 Mb RAM? 64M = 2 26 so you would need 26-input AND gates!

Memory Decoder distancing Suppose we need to store 32 words. How many AND gates will be need in the decoder? How many inputs to each AND gate? What is the size of the memory? This gets expensive – since RAM may reach sizes of 4 GB or more !!!

Memory 2-1/2D Organization of a 64-Word by One-Bit RAM

Memory Example memory module – Adapted from(Texas Instruments, MOS Memory: Commercial and Military Specifications Data Book, Texas Instruments, Literature Response Center, P.O. Box , Denver, Colorado, 1991.) Notice: here the same bits can be used for both address and data in and data out.

Memory Other Elements We will look briefly at one more memory element: ROM (Read only memory) For the other memory: cache, and virtual memory – we will look at the main concepts but not at the hardware details.