8257 use in Intel MDS IOC

Hi, I've been doing some reverse engineering, too. Of the same exact version of firmware you mention. I had to do it in order to build an accurate model of IOC hw in MESS emulator. Anyway, what I understood is that the firmware automatically adapts to either 8237 or 8257. During initialization (@ address 0084) the IOC CPU reads from register 0xfd. This register reads as 00 in case a 8237 is installed (because it's a temporary register that gets reset to 00 at initialization) or ff in case of 8257 (I understand the 8257 leaves the bus floating for registers > 0xf8). The two DMA chips mostly behave the same, apart from different arrangement of registers. Where they do differ a lot is in the "autoload" feature that is only available in 8257. Basically the DMA channel 2 is loaded with data from channel 3 once channel 2 reaches terminal count. In the IOC this feature is used to implement hw scrolling of video because the framebuffer is split in two parts. The second part is actually displayed first whereas the first part is displayed just after the second. Scrolling of video is achieved by simply moving down one row the starting point of second part. The trick is to load channel 2 at raster interrupt with data for second part of framebuffer and channel 3 with data for first part. The DMA hardware automatically switches to supplying data for the first part once the second one ends because of the autoload feature of 8257. See page 4-5 of hardware reference manual (9800556-02). But 8237 has no such feature. My understanding is that an hw modification is needed to IOC board if 8237 is installed (see for example the wiring around the DMA in this picture: https://www.flickr.com/photos/22368471 at N04/16932740579/in/album-72157651893890975/ ). This rewiring (for which I found no documentation) apparently allows the 8237 to "emulate" the autoload feature of 8257. F.Ulivi