Vdc
The VDC (video display controller), officially designated the 8563, is a custom chip designed by Commodore's engineers especially to provide the 80-column display for the 128.
The VDC provides a digital RGBI signal requiring a special monitor, but other than that the fundamentals of its video display are similar to those described for the VIC chip.
The other thing that distinguishes the VDC from the VIC is the VDC's high degree of programmability. Many of the features that are fixed in the silicon of the VIC can be customized on the VDC simply by storing a value in one of its registers. For example, the VDC gives you complete control over the number of rows and columns in the screen display, and even over the number of pixels and scan lines in each character position.
Although the VDC does have a graphic mode (see the entry for bit 7 of register 25/$19), it is primarily used for text displays. In this mode, it uses the same basic elements as the VIC: screen memory, character memory, and attribute memory (which corresponds to the VIC's color memory).
However, all these elements for the VDC appear in a 16K block of memory that is totally separate from the 128's system address space. This separate block is reserved solely for the VDC. Next figure illustrates the configuration of VDC memory.
VDC screen memory holds screen code values identical to the screen codes for the VIC display. For each screen position, the screen code serves as an index into character memory to select the pattern to be displayed in the position. All character patterns are in RAM. The VDC has no character ROM of its own, so the contents of the VIC's character ROM is copied into VDC RAM during system initialization. Redefining characters for the VDC is as simple as storing the new pattern in the proper area of character memory.
Each character definition is 16 bytes long, but because the default character height is eight scan lines, only the first eight bytes are used to hold character pattern information. The remaining eight bytes are generally padded with zeros. To determine the starting address for the definition for any character, use the appropriate formula from the following:
- for uppercase/graphics character set: address = (screen code * 16) + 8192
- for lowercase/uppercase character set: address = (screen code * 16) + 12288
The remaining memory area is attribute memory, which determines the display characteristics for the character specified in screen memory. It is called attribute memory to distinguish it from simple color memory because each attribute memory location specifies more than just the color. Next figure shows the use of each bit of attribute memory
| Value | VDC color |
|---|---|
| 0/$00 | black |
| 1/$01 | dark gray (light black) |
| 2/$02 | dark blue |
| 3/$03 | light blue |
| 4/$04 | dark green |
| 5/$05 | light green |
| 6/$06 | dark cyan |
| 7/$07 | light cyan |
| 8/$08 | dark red |
| 9/$09 | light red |
| 10/$0A | dark purple |
| 11/$0B | light purple |
| 12/$0C | dark yellow |
| 13/$0D | light yellow |
| 14/$0E | light gray (dark white) |
| 15/$0F | white |
Bit 4 selects the flash attribute, causing the character in the corresponding character position to blink at the rate specified in bit 5 of internal register 24/$18.
Bit 5 selects the underline attribute, but the line can be moved to any scan line of the character position.
Bit 6 controls the reverse attribute, which reverses the foreground and background pixels of the character pattern. However, this attribute isn't used by the 128, which instead has reversed character patterns as part of the standard character sets.
Finally, bit 7 selects which of the two character sets will be used. The VIC allows only one of the two character sets to be used at any one time, but the VDC allows ju to select the character set independently for each character position.
The register structure of the VDC chip is rather unusual. It has only two registers visible in the normal system address space. You must go through these registers to access any of the internal functions of the VDC. Next table lists the VDC communications registers visible to the processor. A detailed description of both locations follows.
| Address | Description |
|---|---|
| 54784/$D600 | VDC address/status register |
| 54785/$D601 | VDC data register |