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The Intel 8080 used by these computers was an 8-bit processor, with 16-bit address space, which allowed it access up to 64 KB of memory; .COM executables used with CP/M have a maximum size of 64 KB due to this, as do those used by DOS operating systems for 16-bit microprocessors.
Many 32-bit computers have 32 physical address bits and are thus limited to 4 GiB (2 32 words) of memory. [3] [4] x86 processors prior to the Pentium Pro have 32 or fewer physical address bits; however, most x86 processors since the Pentium Pro, which was first sold in 1995, have the Physical Address Extension (PAE) mechanism, [5]: 445 which allows addressing up to 64 GiB (2 36 words) of memory.
The processor then takes the 32-bit or 16-bit offset and compares it against the segment limit specified in the segment descriptor. If it is larger, a GP fault is generated. Otherwise, the processor adds the 24-bit segment base, specified in descriptor, to the offset, creating a linear physical address.
A 32-bit register can store 2 32 different values. The range of integer values that can be stored in 32 bits depends on the integer representation used. With the two most common representations, the range is 0 through 4,294,967,295 (2 32 − 1) for representation as an binary number, and −2,147,483,648 (−2 31) through 2,147,483,647 (2 31 − 1) for representation as two's complement.
The limit is created by the 32-bit integer limit (2,147,483,647), which is the largest possible integer that can be represented by 32 binary digits. In a computer with a 32-bit architecture, the memory address stored in one of the CPU registers will be limited to this number, thus the number of possible memory locations that can be addressed is ...
It was also available on AMD processors including the AMD Athlon [6] [7] (although the chipsets are limited to 32-bit addressing [8]) and later AMD processor models. When AMD defined their 64-bit extension of the industry standard x86 architecture, AMD64 or x86-64, they also enhanced the paging system in " long mode " based on PAE. [ 9 ]
However, on the 80386, with its paged memory management unit it is possible to protect individual memory pages against writing. [4] [5] Memory models are not limited to 16-bit programs. It is possible to use segmentation in 32-bit protected mode as well (resulting in 48-bit pointers) and there exist C language compilers which support that. [6]
Memory hierarchy also greatly affects processor performance, an issue barely considered in IPS calculations. Because of these problems, synthetic benchmarks such as Dhrystone are now generally used to estimate computer performance in commonly used applications, and raw IPS has fallen into disuse.