1. Memory Systems Slides Created By Kevin Ulin

2. What is Main Memory? 1) Sometimes called primary memory 2) Stores machine instructions for the CPU 3) Stores addresses to other commands and values.

3. Primary vs. Secondary Primary  active data  typically volatile  fast  usable as secondary memory  RAM Secondary  stored data  typically non-volatile  not as fast as primary  hard disk

4. Implementations (Old) Williams-Kilburn Tube  cathode ray tube  measure electrical charges at points on tube  unreliable Drum  paper tape/punchcards  drum rotated while row of heads wait for data

6. Implementations (Old) Delay Line Memory  mercury with piezo crystal transducers  mercury has similar pressure/velocity ratios of piezo crystals  mercury kept at 40 degrees Celsius  later moved on to wires which twisted when it received a bit

8. Implementations (Old) Core Memory (1949 An Wang)  ceramic rings hold magnetic field  hand crafted  Wires running through each ring (x, y, sensor) Twistor Memory (late 1960’s Bell Labs)  similar to core  involved wrapping magnetic tape around wires Bubble Memory (1970s Andrew Bobeck)  tape that held magnetized areas called “bubbles”  stored data even when power was removed

10. Implementations (Current) Integrated Circuits  RAM (Random Access Memory) content accessed in any order flip-flop (SRAM)  maintains information as long as there is power capacitor (DRAM)  needs to be refreshed (leakage)  ROM (Read Only Memory) difficult to write to maintains information after losing power closely resembles hardware ex: BIOS

12. Addresses Static addresses determined at assembly time (globals and static locals) Relative addresses cannot be determined at assembly time (stacks) Addresses might be changed during execution

13. Main Memory’s Purpose/Goals CPU Retrieves information from Main Memory Main Memory must be well designed to prevent CPU from becoming idle. Maintain memory addresses efficiently Must be fast and not put too much pressure on the CPU to manage and maintain it

14. Main Memory Address Handling Techniques 1) Simple Static Addresses 2) Relative Addresses 3) Relocation Register

15. Memory Systems “Multiprogramming”  Multiple programs in memory at one time  When one program is waiting (typically IO), another is running “Uniprogramming”  One program in memory at one time  When the one program is waiting, the CPU is wasted

16. Uniprogramming

17. Static Address Memory is composed of addresses. If we simply define the addresses for each command in memory, one inescapable problem arises. Conflicts in memory. Two Programs could fight for a location.

18. Propagation of Addresses When defining a program you can specify relative addresses for each of the statements.  Header contains relative addresses  Upside: You can start the program at any point in memory.  Downside: Program cannot be moved once started. (Variables may contain addresses)

19. Multiprogramming

21. CPU-I/O Overlap When one program is waiting for input, the CPU runs another Separate Input Processor  Direct Memory Access (DMA)  Cycle Stealing Because both DMA and CPU use Main Memory, DMA’s use is called ‘Cycle Stealing’

22. Timesharing A multiprogramming system Each program given control short periods of time  Timeslice  Only Milliseconds  Regular Intervals

23. Issue with Multiprogramming Programs will start and end at different times Programs will need various sized blocks of memory

25. Relocation Register ‘Logical Address’ goes to an adder, then to memory (Dynamic Address Translation) Register is initially set with load point of program  Ex: You load a program at a load point of 200 into memory, at the same time the relocation register receives the value 200.

26. Relocation Register (Cont.) The program is stored as if it were at address 0. Whenever a logical address is used, the relocation register’s value is added to the logical address, giving the true (physical) address.

28. Relocation Register (Cont.) Prevents memory fragmentation by allowing entire programs to be moved in memory by only changing the register’s value. (In addition to the actual move) Speed cost is very high due to copying entire program.

29. What’s Next? Virtual Memory Cache Memory

30. References Assembly Language And Computer Architecture using c++ and Java by Anthony J. Dos Reis www.wikipedia.com www.columbia.edu (images) www.columbia.edu www.vintagecalculators.com (images) www.vintagecalculators.com www.cedmagic.com (images) www.cedmagic.com www.zdnet.com (images) www.zdnet.com