LAUGHTON ELECTRONICS

Bride of Son of Cheap Video, appendix: Cheap Video Resurfaces

KK Video Overview

"Cheap Video" basics are explained here.

The SCAN Instruction

The KK Cheap Video instruction is called SCAN_NCPX. Its 33h op-code is intercepted and replaced with 20h, the JSR Absolute op-code. The accompanying two-byte operand specifies the destination address, an arbitrary location where pixel data is stored.

The CPU does a jump-to-subroutine to that location and seemingly finds a program of all NOP's. But of course as the CPU bus is getting stuffed with illusory NOPs, the actual memory data is getting copied into a shift register (74HC166) and shipped out as video.

Microcode treats each entire scan as a single instruction, temporarily freezing itself as the string of NOPs is produced. Eventually the freeze is lifted by a periodic signal that's generated by a timer in one of the KK's 65C22 multi-function peripheral chips. (The timer runs at the CRT's horizontal scan rate, roughly 16kHz.) An RTS (60h) replaces the final two NOP's of the scan, and the CPU returns from the subroutine.

It's noteworthy that, since NCP is used, the JSR and RTS are "far" operations, meaning that memory anywhere in the 16 MB space can contain the video bitmap. It's easy to switch between multiple bitmaps if you want to, for zero-overhead updates of the entire screen. And a hi-res bitmap could theoretically be allowed to exceed 64K as long as none of the individual scans has to cross a bank boundary.

Video Hardware

The KimKlone sends a monochrome, composite video signal to an RCA output jack. Usually I used a small, modified TV as my monitor. But there's also an interface for a bitmapped LCD panel I bought on the surplus market.

There's no Character Generator; the display is bitmapped. (If you want characters, the software draws 'em.) I chose not to implement color: the bitmap is a simple, one-bit-per-pixel plane, so each pixel is either black or white.

The vertical and horizontal resolution are widely variable. VSync pulses are generated by software, so controlling vertical res is a simple matter of the varying the values for the loop that surrounds the SCAN_NCPX calls. But to achieve variable horizontal res definitely requires some hardware: the KK uses a variable dot clock and load pulses for the shift register that's trotting out the bits, and also a wait-state generator for the CPU. The heart of that circuit is a programmable divider using a 74AC163 counter running at 80 MHz. The counter reloads on wraparound, with a reload value taken from a 4-bit output port. The port is what allows software to set the divide ratio and thus the dot rate. The duration of the horizontal scans is controlled by a VIA, as mentioned above.