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The predecessor of the 80386 was the Intel 80286, a 16-bit processor with a segment-based memory management and protection system. The 80386 added a three-stage instruction pipeline which it brought up to total of 6-stage instruction pipeline, extended the architecture from 16-bits to 32-bits, and added an on-chip memory management unit. [21]
The primary defining characteristic of IA-32 is the availability of 32-bit general-purpose processor registers (for example, EAX and EBX), 32-bit integer arithmetic and logical operations, 32-bit offsets within a segment in protected mode, and the translation of segmented addresses to 32-bit linear addresses.
In computing, protected mode, also called protected virtual address mode, [1] is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as segmentation, virtual memory, paging and safe multi-tasking designed to increase an operating system's control over application software.
If the paging unit is enabled, addresses in a segment are now virtual addresses, rather than physical addresses as they were on the 80286. That is, the segment starting address, the offset, and the final 32-bit address the segmentation unit derived by adding the two are all virtual (or logical) addresses when the paging unit is enabled.
The paging mechanism uses an on-chip page table with 16Kbyte pages and no access rights checking. [35] V33, V53 [32] RETXA imm8: 0F F0 ib: Return from Extended Address Mode. Jump to an address picked from the IVT using the imm8 argument. Disables paging after reading the IVT but before executing the jump. MOVSPA: 0F 25
Paging allows the CPU to map any page of the virtual memory space to any page of the physical memory space. To do this, it uses additional mapping tables in memory called page tables. Protected mode on the 80386 can operate with paging either enabled or disabled; the segmentation mechanism is always active and generates virtual addresses that ...
Though the 80386 was a 32 bit CPU, it was limited to a 16-bit I/O bus in the case of the Intel Inboard 386/AT and an 8-bit I/O bus in the case of the Intel Inboard 386/PC. Both boards retained 32-bit data and address buses, however.
The Am386 CPU is a 100%-compatible clone of the Intel 80386 design released by AMD in March 1991. It sold millions of units, positioning AMD as a legitimate competitor to Intel , rather than being merely a second source for x86 CPUs (then termed 8086-family ).