Why the original Macintosh had a screen resolution of 512×324

Original Article ↗ Hacker News Discussion ↗

Resolution & Title Confusion

  • Multiple commenters note the HN title used 512×324, but the correct compact Mac resolution is 512×342.
  • Archive.org shows the article briefly contained “324” during an edit, suggesting a live feedback loop between HN and the author.
  • Several note that the menu bar consumed ~20 vertical pixels, leaving about 322 rows for application content.

Why 512×342? Bandwidth, Not Just RAM Size

  • Several argue the key constraint was memory bandwidth, not framebuffer size.
  • The video system alternated RAM access between CPU and display; at 60 Hz, the chosen resolution used roughly half the available DRAM bandwidth during active scan, leaving the rest for the CPU.
  • Commenters reconstruct timing: 512 pixels per line, 342 lines, 60 Hz, and DRAM refresh all fit tightly into the 7.8 MHz memory cycle budget.
  • Others add that 512 is a friendly multiple for efficient graphics on a “32‑bit” architecture, and that Apple likely picked horizontal resolution first, then vertical to approximate square pixels.

Aspect Ratio, Physical Size & 72 dpi

  • Several compute that 512×342 at 72 PPI yields an ~8.5" diagonal, so “9-inch, 72 dpi exact” can’t all be literally true.
  • Clarification: CRT diagonal marketing measured the glass, not the viewable area; repair guides specified a ~7.1"×4.75" visible image, matching ~72 dpi and ~3:2 aspect.
  • There were black borders; some modern owners stretch the image to fill the tube, contrary to original intent.
  • 72 dpi was tied explicitly to 72 typographic points per inch for WYSIWYG desktop publishing.

60 Hz Refresh & Perceived Flicker

  • Discussion branches into whether 60 Hz is really “minimal flicker.”
  • Several users recall 60 Hz CRTs as visibly flickery and preferred 75–85 Hz, especially for text.
  • Others note phosphor persistence, lighting synchronization, and interlacing as key factors.
  • The choice of 60 Hz is linked historically to power-line frequency, TV standards, and engineering convenience; 50 Hz is widely remembered as worse for white backgrounds.

CPU “Bitness” Side Debate

  • Long subthread debates whether the 68000 is “truly” 32‑bit: it has 32‑bit registers but 16‑bit ALUs and bus.
  • Participants conclude “bitness” is a taxonomy issue; from an assembly programmer’s view it largely behaves as 32‑bit, but implementation details blur the label.

Comparisons & Alternate Architectures

  • Commenters contrast the Mac’s shared-memory bitmap with contemporaries using dedicated video chips (Commodore, Atari, consoles) and tile modes to save bandwidth.
  • Others point out Hercules and Lisa resolutions, noting Hercules had non-square pixels and 50 Hz refresh, and that Atari’s high-res monochrome CRTs offered very crisp text.

Design Philosophy & Trade‑off Framing

  • Several emphasize Apple’s explicit “optimize a few areas and design software around them” stance.
  • The 512×342 choice is seen as the result of arithmetic-driven engineering: hit 60 Hz, stay within DRAM timing, maximize usability, match print typography, and keep BOM cost low.
  • Some note that the article still doesn’t pin down a single decisive “why,” leaving aspects—such as why 342 vs a slightly larger line count—ultimately unclear.