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I know a little about assembly, and that there are 4 or 8 or so general purpose registers. How do all the programs on a computer work with just that amount of registers, especially with multithreading and everything?
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In Computer Architecture, the Registers are very fast computer memory which are used to execute programs and operations efficiently. This does by giving access to commonly used values, i.e., the values which are in the point of operation/execution at that time.
Mostly it is limited by instruction set design. Every bit that goes into describing registers has to come at the expense of something else, usually address or opcode space. To get more registers, you would have to give up something else. That is a trade-off that the instruction set designer has to make.
Registers are small amounts of high-speed memory contained within the CPU. They are used by the processor to store small amounts of data that are needed during processing, such as: the address of the next instruction to be executed. the current instruction being decoded.
Nine registers are required for the proposed computer, as shown in Table 2. Some registers (such as AC, MAR, and MBR) may receive data from several multiplexed sources. ...
Multi-threading itself doesn't affect the number of registers in use. When a thread is swapped out, it generally has its registers saved to memory and the next thread to run has those registers loaded up from its previous save.
An example is a system having a thread control block structure (TCB). This structure would contain (while the thread wasn't running), the saved instruction pointer, stack pointer, general purpose registers, floating point registers, thread statistics and so on. In short, everything needed to totally restore the thread to the state it was in when it was swapped out for another thread to run.
And not everything that goes on in a computer is done in registers. Modern compilers can optimise code so that the data items used the most are kept in registers but the vast majority of data is held in memory and only bought into registers when needed.
The best book I've ever read on the subject is Tanenbaum's "Structured Computer Organization" which examines computers in terms of layers, from the digital logic level up to the operating system level, with each level building on the previous.
Aside: my dream is to one day write a book just like this that covers everything, from the quark level up to Emacs :-)
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The other variables and thread stacks are usually stored in protected memory space, where they can be called into registers when needed.
You may want to check out the book The Elements of Computing Systems for a good understanding of how your computer's CPU works. The book is set up as a series of projects where you work up from a NAND gate to a CPU, assembler, simple compiler, and on to a small operating system. It's invaluable in understanding how all your computer's parts fit together.
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