Screen Map

In the Bally Professional Arcade, two bits of RAM define each screen pixel. Therefore, one 8-bit byte of RAM defines four pixels on the screen.

In low-resolution mode, 40 bytes define a line of screen data, providing a horizontal resolution of 160 pixels. The vertical resolution is 102 lines. The areas of the screen defined by the display RAM therefore requires 102 x 40 = 4,080 bytes. The remaining 16 bytes of screen RAM are used for scratchpad. More RAM can be reserved for scratchpad by blanking the screen before the 102nd line.

[High-resolution explanation omitted.]

The first byte of RAM is in the upper lefthand corner of the screen. As RAM addresses increases, the screen position moves in the same direction as the TV scan: from left to right and from top to bottom. The four pixels in each byte are displayed with the least significant pixel (bits 0 and 1) on the right.

[NM:85–6]