IIsi/IIci Memory Usage

This article originally was part of our forthcoming special issue on System 7, but since it really isn’t related to System 7 or software at all, we decided to include it here. If you don’t care about the quirks of on-board video, skip to the next article. Those of you with an a IIsi or IIci should pay attention, though. There will be a quiz. 🙂

When using on-board video, the IIsi and IIci don’t use separate video RAM (VRAM). Instead, they use some of the dynamic RAM (DRAM) that makes up part of the standard system memory. (The Macintosh LC has separate VRAM on its motherboard.) As a default on the IIsi and IIci, 320K of DRAM is reserved for 8-bit color. Switching to black and white mode (with the Options dialog box in the Monitors Control Panel) frees up about 264K of DRAM for system and application use.

A IIsi or IIci user might ask, "Why does my Macintosh IIsi/IIci seem so slow when I have 256 colors or shades of gray turned on?" The explanation is that the video RAM sits in RAM bank A (1 MB of soldered on RAM on the IIsi logic board; four SIMM slots on the IIci). That makes this bank of RAM very busy (the more colors the busier). Consequently, access to anything else in this part of memory is slow. On the IIsi try adjusting the disk cache to at least 384K. As a result, bank A is all video RAM, disk cache and RAM used by the system. This makes the performance with 256 colors almost as good as in black and white. Setting the disk cache higher than 384K does not help much. This will also work with the IIci if you have four 256K SIMMs in RAM bank A. On a IIci, if bank A is not filled with 256K SIMMs (i.e. it is filled with 512K, 1 MB, 2 MB, 4 MB or 16 MB SIMMs) it is harder to keep applications from sharing that busy RAM bank. If possible, try boosting the RAM cache up to approximately the size of the memory in RAM bank A minus 600K and you should get similar results, but this can take away a lot of memory from your applications.

Information from:
Eric Apgar — [email protected]