Sunday, September 6, 2009


There were also four sixteen-bit segment registers (CS, DS, SS, ES, standing for "code segment", "data segment", "stack segment" and "extra segment") that allowed the CPU to access one megabyte of memory in an unusual way. Rather than concatenating the segment register with the address register, as in most processors whose address space exceeded their register size, the 8086 shifted the segment register left 4 bits and added it to the offset address (physical address = 16·segment + offset), producing a 20-bit effective address from the 32-bit segment:offset pair. As a result, each physical address could be referred to by 212 = 4096 different segment:offset pairs. This scheme had the advantage that a small program (less than 64 kilobytes) could be loaded starting at a fixed offset (such as 0) in its own segment, avoiding the need for relocation, with at most 15 bytes of alignment waste. The 16-byte separation between segment bases was known as a "paragraph".
Compilers for the 8086 commonly supported two types of pointer, "near" and "far". Near pointers were 16-bit addresses implicitly associated with the program's code or data segment (and so made sense only in programs small enough to fit in one segment). Far pointers were 32-bit segment:offset pairs. C compilers also supported "huge" pointers, which were like far pointers except that pointer arithmetic on a huge pointer treated it as a flat 20-bit pointer, while pointer arithmetic on a far pointer wrapped around within its initial 64-kilobyte segment.
To avoid the need to specify "near" and "far" on every pointer and every function which took or returned a pointer, compilers also supported "memory models" which specified default pointer sizes. The "small", "compact", "medium", and "large" models covered every combination of near and far pointers for code and data. The "tiny" model was like "small" except that code and data shared one segment. The "huge" model was like "large" except that all pointers were huge instead of far by default. Precompiled libraries often came in several versions compiled for different memory models.
In principle the address space of the x86 series could have been extended in later processors by increasing the shift value, as long as applications obtained their segments from the operating system and did not make assumptions about the equivalence of different segment:offset pairs. In practice the use of "huge" pointers and similar mechanisms was widespread, and though some 80186 clones did change the shift value, these were never commonly used in desktop computers.
According to Morse et al., the designers of the 8086 considered using a shift of eight bits instead of four, which would have given the processor a 16-megabyte address space.[10].

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