S100 Computers - Cromemco STDC

The Cromemco STDC board was an intelligent hard-disk controller, using the ST-506 standard interface to support a wide variety of 5-1/4" Winchester drives. An STDC-controlled drive can have up to 16 heads and 1024 cylinders. As an intelligent disk controller, the STDC needed little attention from the host operating system. With its own Z80A microprocessor, 64K bytes of RAM, and direct access to main memory (DMA), the STDC allowed the host CPU to continue executing instructions during disk read/write operations, relieving the disk I/O "bottleneck" afflicting" many multi-user systems. System performance was further enhanced by the STDC's four full-track data buffers. On each disk read, an entire track is read into one of the data buffers. When subsequent sector requests are on the same track (as is likely), the data is immediately available. If all the buffers were full when a new read request appears, the STDC either discards or writes back to disk the least used of the four buffers (based on frequency of access and time since last use). In addition, the STDC could divide a single disk drive into as many as 32 logical units. Thus, a separate "drive" can be assigned to each user, or the apparent disk size can be matched to a back-up medium. The STDC board was supported by the following operating systems: Cromix-Plus, 68000 Cromix version 20.63 or higher, and Z80 Cromix version 11.27 or higher. To run the STDC and Cromemco WDI-II boards concurrently in the same system, the WDI-II must be modified (refer to appendix C in the manual)

This board would be quite a project to adapt it to a CP/M system BIOS. It was designed to be interrupt driven via a "priority interrupt cable" at the top of the board which connected to the Cromemco FDC and perhaps other boards. There is a good discussion about the board at the vintage-computer forum, see here.

The manual from Cromemco for this board can be obtained here.

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