occurs at .89 MHz. In effect, since the BASIC interpreter runs from ROM, putting the machine in this mode would nearly double the performance of a BASIC program while maintaining video display and DRAM refresh. Of course, this would throw off the software timing loops and I/O operations would be affected. Despite this, however, the "high speed POKE" was used by many CoCo BASIC programs even though it overclocked the hardware in the CoCo, which was only rated for 1 MHz operation. The SAM has no connection to the MPU data bus. As such, it is programmed in a curious manner; its 16-bit configuration register is spread across 32 memory addresses (FFC0-FFDF). Writing even bytes sets that register bit to 0, while writing odd bytes sets it to 1. The value (D7-D0) that is written is ignored. Due to limitations in 40-pin packaging, the SAM contains a duplicate of the VDG's internal 12-bit address counter. Normally this counter's settings are set to duplicate the VDG's display mode. However, this is not required and results in the creation of some new display modes not possible when the VDG is used in a system alone. Instead of the VDG requesting data from RAM by itself, the VDG is "fed" data by the SAM's internal copy of the VDG address counter. This process is called "Interleaved Direct Memory Access" (IDMA) by Motorola and ensures that the processor and VDG always have full access to this shared memory resource with no wait states or contention. There are two versions of the SAM. The early one is labeled MC6883 and/or SN74LS783; the later version is labeled SN74LS785. There are some minor timing differences, but the major difference is the support of an 8-bit refresh counter in the 785 version. This allowed for use of inexpensive 16K by 4-bit and certain 64K by 1-bit DRAMs. Some third-party bank-switching memory upgrades that used 256K DRAMs needed this 8-bit refresh counter to work. VDG The Motorola 6847 is display generator capable of displaying text and graphics contained within a roughly square display matrix 256 pixels wide by 192 lines high. It can display 9 colors: black, green, yellow, blue, red, white, cyan, magenta, and orange. Alphanumeric/Semigraphics display In alphanumeric mode, each character is a 5 dot wide by 7 dot high character in a box 8 dots wide and 12 lines high. This display mode occupies 512 bytes