Processor and memory architecture of computer system

[jackson6612]

Hi

Please have a look on the following linked scanned page:
http://img408.imageshack.us/img408/8782/page102cx.jpg

Please remember that I'm neither a comp. science student nor of science in general. So, please keep you reply as simple and to the point as possible.

Suppose I have the USB drive attached to the computer having some some e-document on it. Here Flash drive is an input device. Right. When I click on that document stored in the flash drive, how would the instruction flow in the context of the diagram shown in the link? I think at first this instruction would flow to the hard disk, then to the RAM, then to the Cache Memory, then to the Control Unit, and if there are some numbers involved, then the Control Unit would send those number operations to Arithmetic Logic unit. Here the inflow path of the instruction ends. And from here the outflow path starts, and ends at some output device such as monitor or printer. Correct?i

Why does "Main Memory (RAM)" have atop three different kinds of memories "ROM, PROM, Flash"? Does this mean that the RAM is divided into three different kinds of memory?

What does those "dots" there in the diagram mean?

Does the instruction from the Main Memory first goes to Control Unit instead of AU?

Please guide me. It would really kind of you. Thanks.

 

[Mark44]

Hi

Please have a look on the following linked scanned page:
http://img408.imageshack.us/img408/8782/page102cx.jpg

Please remember that I'm neither a comp. science student nor of science in general. So, please keep you reply as simple and to the point as possible.

It's probably safe to stop including this disclaimer any more.

Suppose I have the USB drive attached to the computer having some some e-document on it. Here Flash drive is an input device. Right. When I click on that document stored in the flash drive, how would the instruction flow in the context of the diagram shown in the link? I think at first this instruction would flow to the hard disk, then to the RAM, then to the Cache Memory, then to the Control Unit, and if there are some numbers involved, then the Control Unit would send those number operations to Arithmetic Logic unit. Here the inflow path of the instruction ends. And from here the outflow path starts, and ends at some output device such as monitor or printer. Correct?i

There might not be any hard disk activity. The operating system treats memory on the flash drive as just additional memory, so clicking a document stored on the flash drive would cause whatever application is associated with that file type to begin execution. If the associated application is also on the flash drive, the code for that application would be loaded from the flash drive into the cache and so on, and begin executing. OTOH, if the application is stored on the hard drive, then the application code on the hard drive would be loaded into the cache, etc. After the application is up and running, it will start loading the document you clicked into RAM (and then cache) and will probably begin displaying the contents of the document.

Why does "Main Memory (RAM)" have atop three different kinds of memories "ROM, PROM, Flash"? Does this mean that the RAM is divided into three different kinds of memory?

ROM, PROM, and Flash are additional memory types. They are not subsets of RAM. If they were intended to be shown as subsets of RAM, the diagram would show those boxes inside the box for RAM, similar to what is done for the control unit and arithmetic unit (sometimes called ALU for arithmetic/logic unit).

What does those "dots" there in the diagram mean?

They mean "and other general purpose registers."

Does the instruction from the Main Memory first goes to Control Unit instead of AU?

Per the diagram, an instruction goes from memory into the cache, and then into the CPU's control unit (I'm pretty sure). From there it is decoded and executed, although modern processors (of the last 5 to 10 years) there are instruction pipelines that the instruction goes through. I haven't been following CPU architecture as much lately as I used to, so I'm running up against the edge of what I know.

 

[jackson6612]

ROM, PROM, and Flash are additional memory types. They are not subsets of RAM. If they were intended to be shown as subsets of RAM, the diagram would show those boxes inside the box for RAM, similar to what is done for the control unit and arithmetic unit (sometimes called ALU for arithmetic/logic unit).

Thanks a lot, Mark. It was really helpful.

But does a computer really use ROM, PROM, Flash in normally functioning or operation? I believe BIOS is written on some ROM etc. But I don't during the execution of some program these ROM/PROM/Flash are used. It's all about hard disk, RAM, and Cache of a CPU. Isn't it? Please correct me. Thank you.

 

[rcgldr]

But does a computer really use ROM, PROM, Flash in normally functioning or operation? I believe BIOS is written on some ROM etc.

On a PC, MSDOS still uses calls to the BIOS to perform I/O and some memory allocation related functions. Windows NT and later versions of Windows generally stop using the BIOS once they are loaded and running, since they have their own drivers to do the I/O and their own functions for managing memory. Part of the BIOS is stored in ROM, the rest is usually stored in FLASH. Some systems use prom to store code. A flash drive appears to be a hard drive and not an extension of memory on most systems.

 

[jackson6612]

That means in some systems ROM/PROM/EPROM(Flash) still have a role to play in normal functioning of a computer.

ROM once written cannot be rewritten. The same is true of PROM - once programed then cannot be written. Then, what's the real difference between the two - ROM and PROM?

EPROM is just a PROM with the facility of being written again.

Please guide me.

 

[russ_watters]

It was my understanding that the ROM is cached to RAM during booting for faster access.

 

[jackson6612]

Russ, thanks for the input. Yes, ROM is used when a computer is booting; besides this I don't think it's used in normal working when the computer is up and in ready state.

ROM once written cannot be rewritten. The same is true of PROM - once programed then cannot be written. Then, what's the real difference between the two - ROM and PROM?

EPROM is just a PROM with the facility of being written again.

Please guide me.

 

[rcgldr]

Then, what's the real difference between the two - ROM and PROM?

ROM is manufactured with specific data, PROM is programmed by the end user. ROM is cheaper in larger quantities. On a typcial PC, there's a small amount of ROM or PROM, then EPROM or FLASH that can be updated for a BIOS upgade. Older PC's had a jumper that when plugged in at boot up, could restore the EPROM or FLASH from a floppy, but on newer motherboards without floppies, this would have to be done via cd-rom.

It was my understanding that the ROM is cached to RAM during booting for faster access.

Since CPU's have cache, I'm not sure copying ROM to RAM is done on all PCs.

 

[jackson6612]

ROM is manufactured with specific data, PROM is programmed by the end user. ROM is cheaper in larger quantities. On a typcial PC, there's a small amount of ROM or PROM, then EPROM or FLASH that can be updated for a BIOS upgade. Older PC's had a jumper that when plugged in at boot up, could restore the EPROM or FLASH from a floppy, but on newer motherboards without floppies, this would have to be done via cd-rom.

Since CPU's have cache, I'm not sure copying ROM to RAM is done on all PCs.

Hi RCG

Even if a ROM is manufactured with specific data, even them someone would have to program it with data once its circuit board is ready. You cannot simultaneous build a circuit board and program it. Programming would take place at later stage. Then, how does it differ from a PROM?

"could restore the EPROM or Flash from a floppy" - I interpret it to say that EPROM or FLASH could be updated from a floppy once the jumper is plugged in. Do I have it right?

"Since CPU's have cache" - Though I'm a layman without much knowledge of computers, I have already read there is BIOS residing on some ROM type chip. Perhaps, BIOS is built into cache in modern CPU's which have bigger sized caches. In older CPU's, I believe, cache memory was very limited. Please correct me.

Thank you for all the help and your time.

Best wishes
Jackson

 

[rcgldr]

Even if a ROM is manufactured with specific data, even then someone would have to program it with data once its circuit board is ready.

A rom is built as a circuit so that address input = data output. The programming is done during the design of the rom. It's a hardware encoded lookup table, manufactured that way.

A prom is a one time programmable device. Initially all of it's data bits are 1's or 0's (but usually 1's), and then some of those data bits are changed to program the prom.

EPROMS can be erased by exposure to utraviolet light through a window on the EPROM.

EEPROMS are electronically erasable.

FLASH memories involve various technoliges, and are generally faster than EEPROMS.

"could restore the EPROM or Flash from a floppy" - I interpret it to say that EPROM or FLASH could be updated from a floppy once the jumper is plugged in. Do I have it right?

By "restore" I meant in case the EEPROM or FLASH was corrupted, typically due to a power failure during an update.

"Since CPU's have cache"

Modern CPU's have a high speed internal cache memory. Each time data is read from the rom, it gets read into the CPU's cache memory. If that same data is read again, the logic in the CPU determines if that data is still in the internal cache (based on the address of that data), and if so, reads that data from the cache instead from the ROM.

 

[jackson6612]

Thanks a lot for the clarification, RCG.

This link:
http://img408.imageshack.us/img408/8782/page102cx.jpg

shows ROM, PROM, FLASH at the top of RAM. Does a computer really have all these three kinds of memories? I understand most have a FLASH (EEPROM) to update the BIOS. Like the BIOS of my Dell PC could be update which means it has EEPROM. What's the purpose of mentioning all the three kinds of memories at the top? Please let me know. Thanks a lot.

Best wishes
Jackson

 

[rcgldr]

This link:
http://img408.imageshack.us/img408/8782/page102cx.jpg
shows ROM, PROM, FLASH at the top of RAM. Does a computer really have all these three kinds of memories?

Not a typical PC, but that's a generic example of a computer.

A PC usually has a ROM or a EEPROM with the erase disabled in order to allow a "recovery" mode incase the EEPROM or FLASH for the BIOS is corrupted. On older PC's you had to move a jumper, which caused the ROM code to read in floppy and then erase / write the EEPROM or FLASH to initialize the BIOS code and probably set some default values for BIOS settings. Now that motherboards don't support floppies, I assume this is done via cd-rom or the ROM includes a backup of a default BIOS code and data.