Question: A memory-mapped VDT displays the 1024 8-bit characters starting at $00 F000 on a CRT terminal as 16 lines of 64 characters. The address of the top left-hand character is $00 F000, and the address of the bottom right-hand character is $00 F3FF. Design a subroutine to display the ASCII character in DO in the next free position of the display. A cursor, made up of a row count and a column count, points to the next free position into which a character is to be written. As each character is received, it is placed in the next column to the right of the current column counter. Certain characters affect the position of the cursor without adding a new character to the display. These characters are
When the cursor is positioned on the bottom line of the display, a line feed causes all lines to move up one row (i.e.., scroll up). This creates a new, clear bottom line and causes the previous top line to be lost. Construct a subroutine to implement the preceding memory-mapped display. On entry to the subroutine, AO points to the current row position and Al to the column position.