operating the PMBASE value must begin on a 2K boundary. CHBASE $D409 Write SHADOW: CHBAS $02F4 Character Base Address This specifies the page of the base address for the character set. ANTIC Modes 2, 3, 4, and 5 use 128 characters in the character set and require the CHBASE value begin on a 1K boundary. ANTIC Modes 6 and 7 use 64 characters, so the CHBASE value must begin on a 512 byte boundary. The usual default value is $E0hex/224dec for the character set in ROM at $E000hex/57344dec. WSYNC $D40A Write Wait For Horizontal Sync This register allows programs to synchronize to the display. A write to this register halts the 6502 program through the end of the current scanline. This behavior is commonly used during Display List Interrupts to produce clean transitions/changes from one scanline to the next. The value written is unimportant. VCOUNT $D40B Read Vertical Line Counter This register tracks the scan line currently being generated. The value returned is the actual scan line divided by 2. Blank lines generated at the start of the display are included. The value for NTSC will range from 0 to 130 for NTSC, and 0 to 155 for PAL. This value is useful during Display List Interrupts to identify the vertical screen position. PENH $D40C Read SHADOW: LPENH $0234 Light Pen Horizontal Position This contains the horizontal color clock position when the light pen/light gun trigger is pressed. The shadow register is the recommended source for reading this information, since it will be updated during the vertical blank guaranteeing consistent and reliable results. Programs should avoid reading the hardware register directly unless the program is certain the register is read at a time insuring the value is valid. Light guns for the Atari function the same way as light pens with the addition of an optical magnifier allowing the screen to be read from a greater distance. The light pen/light gun input device requires a conventional CRT using a scanning electron beam. The light pen/light gun cannot work with modern LCD TVs and monitors. PENV $D40D Read SHADOW: LPENV $0235 Light Pen Vertical Position This contains the VCOUNT value captured when the light pen/light gun trigger is pressed. The shadow register is the recommended source for reading this information, since it will be updated during the vertical blank guaranteeing consistent and reliable results. Programs should avoid reading the hardware register