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SCSI Accelerator

SCSI Accelerator Icon by Dolf Starreveld
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Year released:

[www].se [ftp].se [mirror].us [mirror].de
scsiaccelerator2.1.sit_.hqx (18.74 KB)
MD5: f6e30ca53162be7bb99f3d9428591cbc
For System 6.x - System 7.0 - 7.6
[www].se [ftp].se [mirror].us [mirror].de
scsiaccelerator7.0.cpt_.hqx (16.55 KB)
MD5: d999c6b987402721104d7f0847a505fc
For System 6.x - System 7.0 - 7.6
[www].se [ftp].se [mirror].us [mirror].de
[www].se [ftp].se [mirror].us [mirror].de
Guides on emulating older applications

SCSI Accelerator is a set of system INITs (extensions) that emerged during the height of the Macintosh Plus in order to patch the Plus' poor implementation of SCSI that results in slow transfer speeds between the Plus and connected devices, in particular hard drives.


Although advertised as having a maximum transfer rate of up to 1.25 MBps (megabytes-per-second), in practice the Plus' transfer rate via SCSI tops out at about 0.26 MBps. This slow transfer rate was originally intended to accommodate the slowest possible SCSI hard drives that might be used with the Plus at the time, but this resulted in very slow transfer rates even with faster devices. In particular, SCSI drives without an internal buffer, intended for use with the Plus, needed at least a 3-to-1 interleave in order to not outpace the sluggish Plus's implementation of SCSI.

While the original creator of SCSI Accelerator is unclear, it was later updated by an individual by the name of Dolf Starreveld, who states in his original Read Me that it was patched in order to fix a long-standing bug that had plagued earlier versions. By 1989, he released v2.1, which made it compatible with System 6.0.x, and he also created an icon for the extension which would only appear at startup if the patch was loaded successfully. (According to Dolf's Read Me, SCSI Accelerator first appeared about a year and a half before he released v2.1, which would place it some time around 1987.)

By 1991, SCSI Accelerator was patched once again, this time by none other than Chad Magendanz (of future ShrinkWrap fame), in order to make it compatible with Pluses running System 7. This latest version was released as "SCSI Accelerator 7".


Selecting which variant to use for your hard drive is unfortunately a matter of trial and error. In brief, the first two digits in each filename refer to the number of pause commands that are inserted into the read and write loops, respectively, that are performed by the SCSI Manager when accessing a device, while the third digit refers to an exponent of the number of "move" commands that are also added to the loops. As a general rule, start with higher numbers, than swap them out for progressively lower numbers in order to see if they might work with your particular configuration.

For a detailed description of SCSI Accelerator and how to select the appropriate variant, please read the instructions included with each download (also included here as PDFs), or see this article from LowEndMac.

Download Info

DL#1: SCSI Accelerator 2.1 (sourced from Jag's House)
DL#2: SCSI Accelerator 7 (sourced from LowEndMac, link above)

Architecture: 68k

This software is only compatible with the Macintosh Plus. It will not do anything on any other Macintosh.

For users of System 7, use SCSI Accelerator 7. System 6 users may use v2.1, but should also be able to use Accelerator 7 as well. (Users of System 5 and earlier may want to stick to v2.1.)