Code: The Hidden Language of Computer Hardware and Software

by Petzold Charles

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...