PC Weasel 2000 : Design

Video emulation is performed by trapping frame-buffer writes to the PC Weasel's video address space, calculating screen coordinates based on the address and converting that to standard ANSI (VT-100, etc.) escape sequences. Character attributes (blink, highlight, underline,color,etc) are also converted to escape sequences. These escape sequences, in addition to the actual character data, are sent to the serial port.

Keyboard input is emulated by reading incoming data from the serial port and mapping it to PC keyboard scan-codes. The <alt> key is accessed with a special character (for example,<ctrl-6>). Provision exists, of course, to actually send the otherwise-trapped "attention" character; typing <ctrl-6> again sends it literally. The attention character can be configured by the end-user.

Some optimizations are performed on video-emulation data by the local processor. For example, the generation of unnecessary cursor addressing information is suppressed for writes to adjacent/consecutive video RAM locations. Although the video-to-serial emulation is complete and usable at all levels of machine operation, it is primarily designed for sub-OS use; that is, for motherboard and peripheral BIOS configuration and bootstrap monitoring.

Example Application:

a) System power-on; board comes out of reset in emulation mode.

b) A write is made to the frame buffer by the BIOS. Data written to video memory is converted to serial output.

c) BIOS goes through normal boot sequence, continuing to send data to MDA board. At this point the user
may interrupt the normal
boot sequence for purposes
of configuration.

d) BIOS loads operating system bootblocks. These are compiled for serial output. Bootblocks write data to COM port address; adapter disconnects RS-232 connector from video emulator's serial output and connects it to the onboard 16550 UART. All input/output is exactly as it would be with a standard PC COM port.

e) Operating system loads and runs in serial console mode.

f) Operating system is rebooted. System resets, goto (b).