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Intel 8086
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Intel 8086 microprocessor is a first member of x86 family of processors. Advertised as a "source-code compatible" with Intel 8080 and Intel 8085 processors, the 8086 was not object code compatible with them. The 8086 has complete 16-bit architecture - 16-bit internal registers, 16-bit data bus, and 20-bit address bus (1 MB of physical memory). Because the processor has 16-bit index registers and memory pointers, it can effectively address only 64 KB of memory. To address memory beyond 64 KB the CPU uses segment registers - these registers specify memory locations for code, stack, data and extra data 64 KB segments. The segments can be positioned anywhere in memory, and, if necessary, user programs can change their position.
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The MIPS and Intel 8086 assembly languages have similarities and differences. One similarity is the addressing modes. Even though Intel 8086 have seven (80386 and 80486 have eight) compared to the five that MIPS have (Computer Organization & Design - Patterson, Hennessy p. 151), some of these addressing modes are similar. For instance, both of these languages have Register Addressing, which is transferring a byte or a word for a source register to a destination register. However, how the instruction is written is different between these two. Let's take addition for an example.
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The new Intel 8086 microprocessor was designed to provide an order of magnitude increase in processing throughput over the older 8080. The processor was to be assembly-language-level-compatible with the 8080 so that existing 8080 software could be reassembled and correctly executed on the 8086. To allow for this, the 8080 register set and instruction set appear as logical subsets of the 8086 registers and instructions. By utilizing a general- register structure architecture, Intel could capitalize on its experience with the 8080 to obtain a processor with a higher degree of sophistication. Strict 8080 compatibility... was not attempted, especially in areas where it would compromise the final design.
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Intel 8086 assembly language corresponding to some simple C code will be examined. Note that convention has 8086 operations showing source and destination operands with destination first (for example: ADD destination, source). Also note the 8086 stack grows downwards towards 0000 (the stack pointer is decremented for a PUSH). Addresses in the 8086 are byte oriented, and each integer type variable occupies 2 bytes. The principles illustrated here are identical when code is examined on different hardware platforms. Same example using TMS320C30
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The ubiquitous Intel 8086 was introduced in 1978. An enormous customer base has ensured that software is still written to the 8086 standard, although many of the tens of millions of original PCs that started it all are today working as plant stands.
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The SDK-86 (System Design Kit) was the first available computer using the Intel 8086 microprocessor. It was sold as a single board kit at a cheaper price than a single 8086 chip! because Intel thought that the success of a microprocessor depends on its evaluation by as many users as possible. All major components were socketed and the kit could be assembled by anyone having a limited technical knowledge thanks to a clear and complete assembly manual. The system could be used with the on-board keyboard and display or connected to a serial video terminal.
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Intel 8086