GEOS SDK TechDocs
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2 8086 Architecture Overview
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2.2 Registers
The memory of the 8086 begins at hexadecimal address 0x00000 and continues (each increment representing one byte) to hexadecimal address 0xFFFFF. Any two consecutive bytes constitute a word. The address of a word is the same as the address of its low byte.
Both the 8088 and 8086 have instructions that access and manipulate words, and both have instructions that access and manipulate bytes. The 8088, however, always fetches a single byte from memory due to its 8-bit bus. The 8086 always fetches two bytes, or a word; if the instruction only operates on a byte, the excess 8 bits will be ignored. This information may be useful to assembly programmers who wish to optimize the performance of their code, but C programmers can ignore it.
As stated earlier, the 8086 can access up to one megabyte of memory. This translates to 2 20 bytes, or 2 20 addresses. However, because the 8086 is designed to do 16-bit arithmetic, it can not directly access all that memory. Instead, an additional mechanism, known as segmentation , is employed.
A segment is a contiguous set of bytes no larger than 64K (2 16 bytes). It must begin on a paragraph boundary (a paragraph is a contiguous block of 16 bytes, the first of which has an address divisible by 16--that is, its address must have zeros for its four least significant bits). This allows the processor to use just 16 bits (leaving off the four zero bits) to access the first byte of a given segment.
To access bytes further into a segment, instructions use not only the 16-bit segment pointer but also a 16-bit offset. Combined, the segment pointer and offset can specify any byte in memory.
Segments may be any size up to 64K. There is no way to specify the exact size of a segment; the processor does not police memory access. Segments may obviously overlap; it is possible, for example, to have two different segments begin 16 bytes apart. Since segments can be any size up to 64K, the two in this example may well overlap.
The addressing mode described above is used in the 8088 and 8086, a large portion of the GEOS market. However, later processors such as the 80386 also employ a protected mode of memory access, in which each process running is relegated a given portion of memory and memory access is policed by the processor. These later processors also can use the segmented mode of the 8086; however, because GEOS applications should be able to run on an 8086 or 8088 machine, they should adhere to the 8086 rules.
GEOS SDK TechDocs
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| 2 8086 Architecture Overview
| 2.2 Registers