There is one particularly interesting extra feature of the 86 status register over that of the A few calculation intensive applications do not verify the presence of the and cannot be run in the There are 25 forms of the MOV move instruction, several of which can simultaneously advance the index registers.
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The last instruction above could equally have microproceessor written in its 'long' version as:. This is not a programming course, but assembly programmers will be able to contrast the with theso it may serve to whet some appetites. It is also necessary because, without local stacks, multiple simultaneous program operation, like foreground and background tasks, would be impossible.
In the first category are the registers, which are used for arithmetic and testing of values and results.
There are 25 forms of the MOV move instruction, several of which micrprocessor simultaneously advance the index registers. When used together, a segment register and a segment offset can define any address in any part of the memory.
If anything less than a segment is addressed, only a single register is needed - if no explicit segment address is given, the value in DS is assumed except in the case of Microprocesor, as noted.
Discontinued BCD oriented 4-bit There are two later processors in this series, the and 8018 These are also 16 bit register and are called:. Because of the 16 bit registers, it is convenient to work within whole segments if at all possible.
Various bits are set or unset as a result of arithmetic operations, comparisons and moves.
When used to indicate a different segment, DS can be pointed to a segment of memory where the program's data is stored, like the currently used part of a document in word processing. The presence or absence of an can be detected in software, so applications which can use the should check for its presence and self configure in it's absence to use overlaid software routines instead.
A few notable personal computers used the You can add a literal value, a value held in memory, indexed or not, or a register's contents to another register. This may vary from one instruction to the next. All memory addressing is based on units of a segment, but the explicit use of the 'extra' nibble is optional i. The series was generally intended for microprocesosr systemsas microcontrollers with external memory. Multiply and microprocessorr also showed great improvement being several times as fast as on the original and multi-bit shifts were done almost four times as quickly as in the To fully appreciate the reasons a small amount of history is called for, together with a little simple arithmetic.
The contents of some registers, like CS and IP are significant to the processor and are excluded from some of the following operations, but within the bounds of common sense; there are almost no limits. When coded into program instructions by these names, all sixteen bits in the register are involved in any operation.
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As usual the smallest base unit is a binary digit, or a bit. The extra segment register, ES, is conventionally used to point to any other area of memory not within a currently identified microprocesxor. Since some packages require more memory than an unexpanded can provide, a memory expansion project has been included later in the book and one is also available from Essential Software.
Code produced specifically for them or compiled on them to use the extra facilities will not run in an machine. In real PCs, which do not directly address more than k, ignoring extended memory systems, which use special mapping, much like sideways RAM does in the BBC Micro the highest address possible is The most notable omission from the chip, when compared with some of its contemporaries, is the absence of support for an maths co-processor.
By convention three of these registers have agreed uses within programs, but the purpose of Cs is fixed by the processor. You should be aware that, while no expansion board will give access to a full megabyte, it can provide more directly usable memory than in any PC- Because of this fact, there are few packages that will take advantage of extra memory over k.
DS is under program control and usually initially defaults to the code segment.