ons of Intel's original depletion-load nMOS implementation, largely aimed at battery-powered devices. On average, the 80286 was reportedly measured to have a speed of about 0.21 instructions per clock on "typical" programs,[7] although it could be significantly faster on optimized code and in tight loops, as many instructions could execute in 2 clock cycles each. The 6 MHz, 10 MHz and 12 MHz models were reportedly measured to operate at 0.9 MIPS, 1.5 MIPS and 2.66 MIPS respectively.[8] The later E-stepping level of the 80286 was free of the several significant errata that caused problems for programmers and operating-system writers in the earlier B-step and C-step CPUs (common in the AT and AT clones).[9] Architecture Simplified 80286 microarchitecture Intel 80286 die shot Intel did not expect personal computers to use the 286.[10] The CPU was designed for multi-user systems with multitasking applications, including communications (such as automated PBXs) and real-time process control. It had 134,000 transistors and consisted of four independent units: the address unit, bus unit, instruction unit and execution unit, organized into a loosely coupled (buffered) pipeline, just as in the 8086. The significantly increased performance over the 8086 was primarily due to the non-multiplexed address and data buses, more address-calculation hardware (most importantly, a dedicated adder) and a faster (more hardware-based) multiplier.[11] It was produced in a 68-pin package, including PLCC (plastic leaded chip carrier), LCC (leadless chip carrier) and PGA (pin grid array) packages.[12] The performance increase of the 80286 over the 8086 (or 8088) could be more than 100% per clock cycle in many programs (i.e., a doubled performance at the same clock speed). This was a large increase, fully comparable to the speed improvements around a decade later when the i486 (1989) or the original Pentium (1993) were introduced. This was partly due to the non-multiplexed address and data buses, but mainly to the fact that address calculations (such as base+index) were less expensive. They were performed by a dedicated unit in the 80286, while the older 8086 had to do effective address computation using its general ALU, consuming several extra clock cycles in many cases. Also, the 80286 was more efficient in the prefetch of instructions, buffering, execution of jumps, and in complex microcoded numerical operations such as MUL/DIV than its predecessor.[11] The 80286 included, in addition to all of the 8086 instructions, all of the new instructions of the 80186: ENTER, LEAVE, BOUND, INS, OUTS, PUSHA, POPA, PUSH immediate, IMUL immediate, and immediate shifts and rotates. The 80286 also added new instructions for protected mode: ARPL, CLTS, LAR, LGDT, LIDT, LLDT, LMSW, LSL, LTR, SGDT, SIDT, SLDT, SMSW, STR, VERR, and VERW. Some of the instructions for protected mode can (or must) be used in real mode to set up and switch to protected mode, and a few (such as SMSW and LMSW) are useful for real mode itself. The Intel 80286 had a 24-bit address bus and was able to address up to 16 MB of RAM, compared to the 1 MB addressability of its predecessor. However, memory cost and the initial rarity of software using the memory above 1 MB meant that 80286 computers were rarely shipped with more than one megabyte of RAM.[11] Additionally, there was a performance penalty involved in accessing extended memory from real mode (in which DOS, the dominant PC operating system until the mid-1990s, ran), as noted below. Features Siemens 80286 (10 MHz version) IBM 80286 (8 MHz version) Protected mode The 286 was the first of the x86 CPU family to support protected virtual-address mode, commonly called "protected mode". In addition, it was the first commercially available microprocessor with on-chip MMU capabilities (systems using the contemporaneous Motorola 68010 and NS320xx could be equipped with an optional MMU controller). This would allow IBM compatibles to have advanced multitasking OSes for the first time and compete in the Unix-dominated server/workstation market. Several additional instructions were introduced in protected mode of 80286, which are helpful for multitasking operating systems. Another important feature of 80286 is prevention of unauthorized access. This is achieved by: Forming different segments for data, code, and stack, and preventing their overlapping. Assigning privilege levels to each segment. Segments with lower privilege levels cannot access segments with higher privilege levels. In 80286 (and in its co-processor Intel 80287), arithmetic operations can be performed on the following different types of numbers: unsigned packed decimal, unsigned binary, unsigned unpacked decimal, signed binary, floating-point numbers (only with an 80287).