The MC6883 SAM makes several parts of an early Color Computer or Dragon 32 work together. SAM stands for synchronous address multiplexer. It supplies addresses and timing so the processor, video hardware and dynamic RAM can share a working memory system.
SAM and the video generator have separate jobs. Motorola’s system diagram puts SAM between the 6809, RAM and the MC6847 Video Display Generator. The VDG turns data into video. SAM helps get the right data to it, while also serving processor accesses and refreshing DRAM.
One RAM, several jobs
Dynamic RAM needs periodic refresh to retain its contents. The processor needs memory for instructions and data. The VDG needs a stream of display bytes at the right time. Those requests have to fit the same hardware’s access schedule.
SAM multiplexes addresses for the memory and supplies the timing needed by the surrounding system. Its programmable settings include RAM size, video addressing mode and video-memory offset. A changed video offset can make the display read from a different region without copying the whole picture. That changes where bytes are fetched, rather than changing what the VDG thinks the bytes mean. (Motorola, feature list and system block diagram.)
A mode has more than one setting
The division matters when setting graphics modes. The video generator’s interpretation and SAM’s pattern of memory addresses must agree. A suitable VDG mode paired with an unsuitable SAM setting will not fetch the intended screen correctly.
Duncan Smeed and Ian Sommerville’s Inside the Dragon explains the relationship through the machine’s graphics and its SAMMOD routine. It is a more useful guide to a Dragon program than treating the chip’s feature list as a complete recipe: the computer adds ROM routines, a memory map and particular display conventions.
Clock speed has consequences
SAM can select processor rates derived from the crystal, including address-dependent operation. That does not mean every machine can use every setting safely in every display mode. The video and memory timing still have to work together. The Dragon 32 entry covers the contemporary debate about speed-up settings and their limits.
Motorola also lists a 96KB addressing range in this SAM data sheet. That describes the memory-system circuitry; it does not give an ordinary 6809 program a flat 96KB address space. The CPU still issues 16-bit addresses. Keeping those two descriptions separate prevents a support chip’s capacity from becoming an invented processor feature.
Sources
- Motorola, SN74LS783/MC6883 and SN74LS785 Synchronous Address Multiplexer, ADI-595R2, 1984 copyright. Original data sheet.
- Duncan Smeed and Ian Sommerville, Inside the Dragon, Addison-Wesley, 1983, pp. 154–157, 162 and 171–172: video hardware and the SAMMOD routine.