Television Protocol
(The TV picture according to Atari)
For the purposes of Stella programming, a single NTSC television “frame” consists of 262 horizontal lines, and each line is divided by 228 clock counts (~3.58 MHz NTSC). The actual TV picture is drawn line by line from the top down 60 times a second, and actually consists of only a portion of the entire “frame”. A typical frame will consists of 3 vertical sync (VSYNC) lines(to signal the TV set to start a new frame), 37 vertical blank (VBLANK) lines, 192 TV picture lines, and 30 overscan lines. Atari’s research has shown that this pattern will work on all types of TV sets. Each scan lines starts with 68 clock counts of horizontal blank (not seen on the TV screen) followed by 160 clock counts to fully scan one line of TV picture. When the electron beam reaches the end of a scan line, it returns to the left side of the screen, waits for the 68 horizontal blank clock counts, and proceeds to draw the next line below.
All horizontal timing is taken care of by hardware, but the microprocessor must “manually” control vertical timing to signal the start of the next frame. When the last line of the previous frame is detected, the microprocessor must generate 3 lines of VSYNC, 37 lines of VBLANK, 192 lines of actual TV picture, and 30 lines of overscan. Fortunately, both VSYNC and VBLANK can simply be turned on and off at the appropriate times, freeing the microprocessor for other activities during their execution.
The actual TV picture is drawn one line at a time by having the microprocessor enter the data for that line into the Television Interface Adaptor (TIA) chip, which then converts the data into video signals. The TIA can only have data in it that pertains to the line being currently drawn, so the microprocessor must be “one step ahead” of the electron beam on each line. Since one microprocessor machine cycle occurs every 3 clock counts, the programmer has only 76 machine cycles per line (228/3 = 76) to construct the actual picture (actually less because the microprocessor must be ahead of the raster). To allow more time for the software, it is customary (but not required) to update the TIA every two scan lines. The portion of the program that constructs this TV picture is referred to as the “Kernel”, as it is the essence or kernel of the game.
In general, the remaining 70 scan lines (3 for VSYNC, 37 for VBLANK, and 30 for overscan) will provides 5,320 machine cycles (70 lines x 76 machine cycles) for housekeeping and game logic. Such activities as calculating the new position of a player, updating the score, and checking for new inputs are typically done during this time.
Related
see Clock Speeds