Screen Map

In the Bally 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 (aka “consumer”) 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.

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.

In consumer mode, each line is scanned twice per field (the second time immediately after the first), to give 204 active lines of video. The remaining 58 1⁄2 lines of video (each NTSC field is 262 1⁄2 lines) is made up of blanking and sync lines. Each frame consists of two fields, scanned in an interlaced format, at 60 fields (or 30 frames) per second.

[NM:85–6]

Reference: Tony Miller, A Description of the Bally Professional Arcade Video Hardware and Associated Coin-Operated Hardware (2001)