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  1. I have a question. For the 32 bit CPU, the maximal memory address it can have is 2^32 bit = 4GB. So does it mean 32 bit CPU computer only needs a 4GB RAM? If it is provided a 8GB RAM, it can only use up to 4GB RAM.

  2. I try to answer the 1st question below. But I have anther question, for 40bit AB and 32 bit OS, the logical address is up to 2^32 = 4GB, then how can it use the 40bit AB?

Yuanfei Bi
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1 Answers1

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Modified according to the comments,

32 bit computer usually means the register is 32bit, it has noting to do with DB. If the width of DB is 32bit, it can transfer 32 bit data one time.

The width of AB is often 32 bit, but it can be 40 bit as well. The width of AB determines the memory space it can use. That is the limit from the hardware. And the actual memory it can use is further constrained by the OS.

Yuanfei Bi
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  • Being 32-bit usually means having 32-bit (integer) registers. It does *not* mean anything about the width of the data bus. e.g. P5 Pentium is a 32-bit CPU but has a 64-bit data bus, and can load or store a 64-bit `double` atomically. Also, it has a cache so the data bus is actually used for burst transfers of 32-byte cache lines. – Peter Cordes May 27 '20 at 21:19
  • Using more than 32-bit address space depends on the ISA. For example, 32-bit x86 with PAE (wider physical addresses in page table entries, but still 32-bit virtual addresses). Or PowerPC had a way to extend logical addresses beyond 32-bit. – Peter Cordes May 27 '20 at 21:20