Vdg

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.

Motorola's MC6847 Video Display Generator (VDG) chip was released about the same time as the joint venture started and it has been speculated that the VDG was actually designed for this project.

Again the colorset input to the VDG determined which colors were used.

A new VLSI ASIC, called (officially) the Advanced Color Video Chip (ACVC) or (unofficially) the Graphics Interrupt Memory Enhancer (GIME), integrated the functions of the SAM and VDG while enhancing the capabilities of both.

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.

It is not possible to reliably display 256 dots across the screen due to the limitations of the NTSC signal and the phase relationship between the VDG clock and colorburst frequency.

Normally this counter's settings are set to duplicate the VDG's display mode.

This enhanced VDG allowed the use of lower case characters and the ability to change the text screen border color.

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.

Unfortunately the VDG internally can power up on either the rising or falling edge of the clock, so the bit patterns that represent orange and blue are not predictable.

Very late in the CoCo 2 production run, an enhanced VDG was available.

Writing even bytes sets that register bit to 0, Writing to odd bytes sets it to 1. Due to limitations in 40-pin packaging, the SAM contains a duplicate of the VDG's internal 12-bit address counter.