Random access memory Sequential circuits all depend upon the presence of memory A flip flop can store one bit of information A register can store a single word typically 32 64 bits Random access memory or RAM allows us to store even larger amounts of data Today we ll see The basic interface to memory How you can implement static RAM chips hierarchically This is the last piece we need to put together a computer 11 3 2003 Random Access Memory 1 Introduction to RAM Random access memory or RAM provides large quantities of temporary storage in a computer system Remember the basic capabilities of a memory It should be able to store a value You should be able to read the value that was saved You should be able to change the stored value A RAM is similar except that it can store many values An address will specify which memory value we re interested in Each value can be a multiple bit word e g 32 bits We ll refine the memory properties as follows A RAM should be able to Store many words one per address Read the word that was saved at a particular address Change the word that s saved at a particular address 11 3 2003 Random Access Memory 2 Picture of memory You can think of computer memory as being one big array of data The address serves as an array index Each address refers to one word of data You can read or modify the data at any given memory address just like you can read or modify the contents of an array at any given index If you ve worked with pointers in C or C then you ve already worked with memory addresses 11 3 2003 Random Access Memory Address 00000000 00000001 00000002 FFFFFFFD FFFFFFFE FFFFFFFF Data 3 Block diagram of RAM 2k x n memory k n ADRS DATA CS WR OUT n CS WR 0 1 1 x 0 1 Memory operation None Read selected word Write selected word This block diagram introduces the main interface to RAM A Chip Select CS enables or disables the RAM ADRS specifies the address or location to read from or write to WR selects between reading from or writing to the memory To read from memory WR should be set to 0 OUT will be the n bit value stored at ADRS To write to memory we set WR 1 DATA is the n bit value to save in memory This interface makes it easy to combine RAMs together as we ll see 11 3 2003 Random Access Memory 4 Memory sizes We refer to this as a 2k x n memory There are k address lines which can specify one of 2k addresses Each address contains an n bit word 2k x n memory k n ADRS DATA CS WR OUT n For example a 224 x 16 RAM contains 224 16M words each 16 bits long The RAM would need 24 address lines The total storage capacity is 224 x 16 228 bits 11 3 2003 Random Access Memory 5 Size matters Memory sizes are usually specified in numbers of bytes 8 bits The 228 bit memory on the previous page translates into 228 bits 8 bits per byte 225 bytes With the abbreviations below this is equivalent to 32 megabytes K M G Prefi x Base 2 Kilo 210 1 024 Mega 220 1 048 576 Giga 230 1 073 741 824 Base 10 103 1 000 106 1 000 000 109 1 000 000 000 To confuse you RAM size is measured in base 2 units while hard drive size is measured in base 10 units In this class we ll only concern ourselves with the base 2 units 11 3 2003 Random Access Memory 6 Typical memory sizes Some typical memory capacities PCs usually come with 128 256MB RAM PDAs have 8 64MB of memory Digital cameras and MP3 players can have 32MB or more of storage Many operating systems implement virtual memory which makes the memory seem larger than it really is Most systems allow up to 32 bit addresses This works out to 232 or about four billion different possible addresses With a data size of one byte the result is apparently a 4GB memory The operating system uses hard disk space as a substitute for real memory 11 3 2003 Random Access Memory Address 00000000 00000001 00000002 FFFFFFFD FFFFFFFE FFFFFFFF Data 7 Reading RAM To read from this RAM the controlling circuit must Enable the chip by ensuring CS 1 Select the read operation by setting WR 0 Send the desired address to the ADRS input The contents of that address appear on OUT after a little while Notice that the DATA input is unused for read operations 2k x n memory k n 11 3 2003 ADRS DATA CS WR OUT Random Access Memory n 8 Writing RAM To write to this RAM you need to Enable the chip by setting CS 1 Select the write operation by setting WR 1 Send the desired address to the ADRS input Send the word to store to the DATA input The output OUT is not needed for memory write operations 2k x n memory k n 11 3 2003 ADRS DATA CS WR OUT Random Access Memory n 9 Static memory How can you implement the memory chip There are many different kinds of RAM We ll start off discussing static memory which is most commonly used in caches and video cards Later we mention a little about dynamic memory which forms the bulk of a computer s main memory Static memory is modeled using one latch for each bit of storage Why use latches instead of flip flops A latch can be made with only two NAND or two NOR gates but a flip flop requires at least twice that much hardware In general smaller is faster cheaper and requires less power The tradeoff is that getting the timing exactly right is a pain 11 3 2003 Random Access Memory 10 Starting with latches To start we can use one latch to store each bit A one bit RAM cell is shown here Since this is just a one bit memory an ADRS input is not needed Writing to the RAM cell When CS 1 and WR 1 the latch control input will be 1 The DATA input is thus saved in the D latch Reading from the RAM cell and maintaining the current contents When CS 0 or when WR 0 the latch control input is also 0 so the latch just maintains its present state The current latch contents will appear on OUT 11 3 2003 Random Access Memory 11 My first RAM We can use these cells to make a 4 x 1 RAM Since there are four words ADRS is two bits Each word is only one bit so DATA and OUT are one bit each Word selection is done with a decoder attached to the CS inputs of the RAM cells Only one cell can be read or written at a time Notice that the outputs are connected …
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