Sunday, June 12, 2005
What's your byte-sex?
Microprocessors have data registers. These are analogous to little scratch-pads on your desktop. When the computer needs to do some manipulation of data, it pulls that data out of memory and puts it into these little scratch pads. When the computer is done with that data, yet wants to save it for later use, it writes the data on the scratch pad out to memory.
Early microprocessors had 8-bit, or byte-sized, data registers. This was generally convenient, because memory is considered to be just a very huge array of bytes. However, when the first 16-bit microprocessors were designed, the size of the scratch pads, or data registers, was twice the size of a single, addressable piece of memory.
This introduced a design problem. When you read a 16-bit value from a byte-addressable data store, like memory, which byte do you load first? You have two choices. You can load the most-significant byte first (known as big-endian), or you can load the least-significant byte first (known as little-endian). The difference between big-endian and little-endian systems is often referred to as "byte-sex". It's a term I've used before, and, for those who've wondered what it means, now you know.
As luck would have it, the two most active designers of microprocessors at the time, Intel and Motorola, answered that question in opposite ways. I'm not well-versed in VLSI design, but I have little doubt that each group of chip designers had legitimate reasons for the choices they made. None of that is important here. What is important is that, way back in those days, Apple chose Motorola processors while IBM chose Intel. Thus began the great endian divide.
Early microprocessors had 8-bit, or byte-sized, data registers. This was generally convenient, because memory is considered to be just a very huge array of bytes. However, when the first 16-bit microprocessors were designed, the size of the scratch pads, or data registers, was twice the size of a single, addressable piece of memory.
This introduced a design problem. When you read a 16-bit value from a byte-addressable data store, like memory, which byte do you load first? You have two choices. You can load the most-significant byte first (known as big-endian), or you can load the least-significant byte first (known as little-endian). The difference between big-endian and little-endian systems is often referred to as "byte-sex". It's a term I've used before, and, for those who've wondered what it means, now you know.
As luck would have it, the two most active designers of microprocessors at the time, Intel and Motorola, answered that question in opposite ways. I'm not well-versed in VLSI design, but I have little doubt that each group of chip designers had legitimate reasons for the choices they made. None of that is important here. What is important is that, way back in those days, Apple chose Motorola processors while IBM chose Intel. Thus began the great endian divide.
