Code: The Hidden Language of Computer Hardware and Software

by Petzold Charles

What makes the difference is the conditional jump. Controlled repetition or looping is what separates computers from calculators.

“We may say that the Analytical Engine weaves algebraical patterns just as the Jacquard-loom weaves flowers and leaves.”

These design decisions were such an important evolutionary step that today we speak of von Neumann architecture. The computer that we built in the last chapter was a classic von Neumann machine. But with von Neumann architecture comes the von Neumann bottleneck. A von Neumann machine generally spends a significant amount of time just fetching instructions from memory in preparation for executing them. You’ll recall that the final design of the Chapter 17 computer required that three-quarters of the time it spent on each instruction be involved in the instruction fetch.

But semiconductors can be doped, which means that they’re combined with certain impurities. One type of impurity adds extra electrons to those needed for the bond between the atoms. These are called N-type semiconductors (N for negative). Another type of impurity results in a P-type semiconductor. Semiconductors can be made into amplifiers by sandwiching a P-type semiconductor between two N-type semiconductors. This is known as an NPN transistor, and the three pieces are known as the collector, the base, and the emitter.

Here’s another interesting difference between the 8080 and the 6800: In both microprocessors, the instruction LDA loads the accumulator from a specified memory address. In the 8080, for example, the following sequence of bytes: will load the accumulator with the byte stored at memory address 347Bh. Now compare that with the 6800 LDA instruction using the so-called 6800 extended addressing mode: This sequence of bytes loads accumulator A with the byte stored at memory address 7B34h. The difference is subtle. You expect the opcode to be different, of course: 3Ah for the 8080 and B6h for the 6800. But the two microprocessors treat the address that follows the opcode differently. The 8080 assumes that the low-order byte comes first, followed by the high-order byte. The 6800 assumes that the high-order byte comes first! This fundamental difference in how Intel and Motorola microprocessors store multibyte values has never been resolved. To this very day, Intel microprocessors continue to store multibyte values with the least-significant byte first (that is, at the lowest memory address), and Motorola microprocessors store multibyte values with the most-significant byte first. These two methods are known as little-endian (the Intel way) and big-endian (the Motorola way). It might be fun to argue over which method is better, but before you do so, be aware that the term Big-Endian comes from Jonathan Swift’s Gulliver’s Travels and refers to the war between Lilliput and Blefuscu ove...

To run, CP/M must be loaded from the disk into memory. The ROM in a computer that uses CP/M need not be extensive. All the ROM needs to contain is a small piece of code known as a bootstrap loader (because that code effectively pulls the rest of the operating system up by its bootstraps). The bootstrap loader loads the very first 128-byte sector from the diskette into memory and runs it. This sector contains code to load the rest of CP/M into memory. The entire process is called booting the operating system.