Expert C Programming

the chip that got Intel started, the 8080 was the chip that made its fortune, boosting the company's
turnover to more than $1 billion annually and placing it high on the Fortune 500 list.

The 8085 processor took advantage of advances in integration technology to squeeze a three chip
combination onto one. In essence, it was an 8080 combined with the 8224 clock driver and the 8228
controller, all on a single chip. Although its internal data bus was still 8-bit, it used a 16-bit address
bus, so it could address 2^16 or 64 Kbytes of memory.

Introduced in 1978, the 8086 improved on the 8085 by allowing 16 bits on the data bus and 20 bits on
the address bus permitting a massive (at the time) 1 Mbyte of possible memory. In an unusual design
decision, the addresses are formed by overlapping two 16-bit words to form a 20-bit address, rather
than concatenating them to form a 32-bit address (see Figure 7-2). The 8086 was not 8085-compatible
at the instruction-set level, but assembler macros helped convert programs to the newer architecture
easily.

Figure 7-2. How the Intel 8086 Addresses Memory

Its irregular addressing scheme allowed the 8086 to run 8085-ported code more simply; once the
segment registers were loaded with a fixed value they could be ignored, allowing the 16-bit addresses
of the 8085 to be used directly. The design team rejected the idea of forming addresses by
concatenating the segment word; that would provide 32 bits of addressing or 4 Gbytes, which was an
impossibly large amount of memory at the time.

Now that this basic addressing model was laid down, all subsequent 80x86 processors had to follow it
or give up compatibility. If the 8080 was the chip that brought Intel to prominence, the 8086 was the
chip that kept it there. We'll probably never know exactly why IBM selected the Intel 8088 (an 8-bit
sibling of the 8086) as the CPU for its new PC back in 1979, in the face of so many technically
superior alternatives from companies like Motorola or National Semiconductor. By selecting an Intel
chip, IBM made Intel's fortune for the next two decades, just as IBM also made Microsoft's fortune by
selecting MS-DOS as the operating executive. Ironically, in August 1993 Intel's stock valuation of
$26.6 billion rose above IBM's stock valuation of $24.5 billion, and Intel eclipsed IBM as the most
valuable electronics company in America.