The Cosmac Elf 2000 has 2 video options, the ’onboard’ CDP 1861 or the 80 Column Text Video Board running the intel 8275. Next to this also a Video Terminal like the DEC VT52 or a PC running terminal emulation can be connected.
For details on the CDP 1861 please see the data sheets for this chip or the ’summary’ on the CDP 1861 page. The Elf 2000 controls the CDP 1861 via OUT 1 to switch the display off, OUT 2 to switch the display on and EF1 to indicate the display status.
The Elf 2000 firmware has a CDP 1861 test function, started via command TE PIX (This command can be given if a video terminal is connected to the Elf 2000). The test function uses the most common CDP 1861 resolution of 64 x 32 with interrupt routine entry point at @9072 and display memory at @9500-@95FF (ROM, showing the ‘famous’ enterprise picture).
The display status is an active low output signal occurring for a period of four horizontal cycles prior to the beginning and end of the 128 line display window. The signal is used by the firmware interrupt routine to indicate the last ’row’ of the display area.
The 8275 retrieves characters from memory and displays them on a row by row basis. The 8275 has two row buffers. While one row buffer is being used for display, the other is being filled with the next row of characters to be displayed. The number of display characters per row and the number of character rows per frame are software programmable.The 8275 requests DMA to fill the row buffer that is not being used for display. DMA burst length and spacing is programmable.
The 8275 displays character rows one line at a time. The number of lines per character row, the underline position, and blanking of top and bottom lines are programmable.
For additional details on the intel 8275 please see the data sheet.
The Elf 2000 controls the 8275 via, OUT 5 to write the command, INP 5 to read the status and OUT 1 / INP 1 to write / read a parameter. EF1 is connected to the vertical retrace.
The Elf 2000 uses the 8275 with 8x10 characters, 80 characters per row and 24 rows per screen.
Note that the video mode ’highlight’ is not supported in the HW and the invisible field atribute mode is not supported by the Elf 2000 firmware.