BASIC starts out in OS graphics mode 0 which displays 40 characters across by 24 rows. This is IR mode 2 with a standard screen width. The OS sets up the display list near the top of RAM with room for the character names at the top of RAM. On a 32 K-byte machine, the display list would start at hex 7C20. The next three bytes are hex 70's to create 24 blank lines. The next byte is a hex 42. The 4 tells the hardware to reload the memory scan counter with the following address (7C40). This is the address of the data to be displayed. The 2 tells the hardware to display one line of IR mode 2 characters. The next 23 bytes specify 23 more lines of mode 2 characters. Hex 41 is the code for jumping and waiting until the end of the next vertical blank. The address to jump to is 7C20, the start of the display list. The next 960 bytes are the list of characters to be displayed, 40 bytes per line. The OS must set up the display list pointer (DLISTH and DLISTL) to the starting address of the display list (7C20). It also sets CHBASE to the MSB of the address of the character set (E0).
This is a simple example because only one mode is used and the memory scan counter is only loaded at one point. It is possible to have different modes on different lines, change character sets and colors, etc., as shown in the example in Section IV.
OS Mode 0 Display List (40 chars x 24 lines)
Address (hex) Data (hex) 7C20 70
70
7024 blank lines 42
40
7Creload memory scan counter
with 7C40, IR mode 22
2
2
...
2
2
223 more IR mode 2 instructions 41
20
7CJump back to 7C20 and
wait for end of vertical blank.7C40 960 bytes of display data (character names)