Just to round out the discussion, the identical concept is part of the code in RT-11, specifically DX.MAC and DY.MAC, which are the RX01 and RX02 device drivers, respectively. While I never had a reason to look closely at the DXX.SYS device driver, I did modify the DYX.SYS or RX02 device driver. The interleaving is primarily required to allow time for the hardware to transfer the silo to the user buffer and then initiate the next read. Otherwise, it would be possible to read only one sector per revolution of the floppy media. However, there is an 18 bit hardware address limitation for the DEC RX02 drive in addition to other RX02 compatible drives such as the DSD (Data System Design) models. This means that the DEC version of the DYX.SYS device driver is able to support user buffers only up the 1/4 MegaByte of physical memory. Up to V04.00 of RT-11, this was sufficient. Starting with V05.00 of RT-11, the full 4 MegaBytes of physical memory was supported by RT-11 and a user buffer in an area of physical memory beyond 1/4 MegaByte could not use the DEC device driver for the RX02. On the other hand, DYX.SYS can be modified to include a bounce buffer which can be used to hold the contents of the hardware silo for the very short period of time required, although the speed of the PDP-11/23 is not fast enough to support the bounce buffer by a simple transfer of the silo to the bounce buffer followed by a copy of the bounce buffer to the user buffer at an address above 1/4 MegaByte of physical memory. I found this out when it became obvious (after a successful test with the PDP-11/73) when the transfer rate slowed by an unacceptable degree. The simple solution was to initiate the next read into the silo immediately after the contents of the silo were transferred to the bounce buffer and then copy the bounce buffer to the user buffer. That change in the order allowed the PDP-11/23 to keep up with the transfer using the standard interleave. I doubt that anyone uses the RX02 very much under RT-11 on real hardware, but the code is available if anyone has a user program which uses extended memory above 1/4 MegaByte and employs the RX02 DYX.SYS device driver. Note that the bounce buffer and the extra code are both in lower memory since I never used it enough to make it worth my while to take advantage of extended memory below the 1/4 MegaByte area. I have not tested the code under either SIMH or E11, but it should work there just as well. Just a bit of information about using the RX02 DYX.SYS device driver under RT-11. Jerome Fine