Why the Original Macintosh Had a Screen Resolution of 512×324

Summary

Many classic Macs came with — or supported — displays running at 512×384 pixels, but many compact Macs, ranging from the original 1984 machine up through 1991’s Macintosh Classic II had built-in CRTs running at 512×342 pixels. That covers all black-and-white compact Macs with a 9-inch screen. The later Color Classic and Color Classic II used a 10-inch CRT at a full 512×384. This came up when I joined John Gruber on The Talk Show. At one point in the show, I rattled off the original Mac’s resolution as being 512×384. Except… it wasn’t. The original Mac screen ran at 512×342. I remembered the right number and corrected myself a moment later, but given the name of this website, it was pretty embarrassing. This set me off on a journey to understand why Apple made this decision. Why were the displays on early Macs 42 pixels shorter in height than later ones? After doing a lot of reading, there are several factors to consider when trying to answer this question. Memory The original Mac had a mere 128 kilobytes of memory. The photo of the original Macintosh in this blog post is 604 KB in size, some 4.7x larger than the entire memory footprint of the 1984 machine. Of course, Apple would improve this with later models, but many design decisions made to accommodate the original Mac would forward for years. Over at Folklore.org, Andy Hertzfeld wrote a great post walking through several early versions of what would become the Macintosh, including ones with even less memory: In the beginning of 1982, the original 68000 design was more than a year old, and the software was nowhere near finished, so Burrell [Smith] was afraid some of the trade-offs of the original design were no longer current. He used the expansive canvas of a custom chip, where additional logic was almost free, to update the architecture. The most important decision was admitting that the software would never fit into 64K of memory and going with a full 16-bit memory bus, requiring 16 RAM chips instead of 8. The...