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Below is the full 8086/8088 instruction set of Intel (81 instructions total). [2] These instructions are also available in 32-bit mode, in which they operate on 32-bit registers (eax, ebx, etc.) and values instead of their 16-bit (ax, bx, etc.) counterparts.
The last segment, FFFFh (65535), begins at linear address FFFF0h (1048560), 16 bytes before the end of the 20 bit address space, and thus, can access, with an offset of up to 65,536 bytes, up to 65,520 (65536−16) bytes past the end of the 20 bit 8088 address space. On the 8088, these address accesses were wrapped around to the beginning of ...
Die of AMD 8088. The 8088 was designed at Intel's laboratory in Haifa, Israel, as were a large number of Intel's processors. [9] The 8088 was targeted at economical systems by allowing the use of an eight-bit data path and eight-bit support and peripheral chips; complex circuit boards were still fairly cumbersome and expensive when it was released.
The 16-bit Intel 8088 and Intel 8086 supported 20-bit addressing via segmentation, allowing them to access 1 MiB rather than 64 KiB of memory. All Intel Pentium processors since the Pentium Pro include Physical Address Extensions (PAE) which support mapping 36-bit physical addresses to 32-bit virtual addresses.
The Intel 8088 CPU, used in the original IBM PC, was able to address 1 MB (2 20 bytes), since the chip offered 20 address lines. In the design of the PC, the memory below 640 KB was for random-access memory on the motherboard or on expansion boards, and it was called the conventional memory area.
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]
On the 8086, 8088, and 80186, the result of an effective address that overflows 20 bits is that the address "wraps around" to the zero end of the address range, i.e. it is taken modulo 2^20 (2^20 = 1048576 = 0x100000). However, the 80286 has 24 address bits and computes effective addresses to 24 bits even in real mode.
Intel and the whole x86 ecosystem needed 64-bit memory addressing if x86 was to survive the 64-bit computing era, as workstation and desktop software applications were soon to start hitting the limits of 32-bit memory addressing. However, Intel felt that it was the right time to make a bold step and use the transition to 64-bit desktop ...