86:Ports:00

Synopsis

Port Number: 00h

Function: LCD DMA Base Address

This port controls the base address used as video memory by LCD DMA.

Read Values

Because this port is write-only, its read behavior is the same as Unused Banks.

Write Values

• Bit 0-5: Bits 8-13 of LCD base address. The lower 8 bits of the address are always 0, & it is always in RAM page 0.
• Bit 6-7: Unused.

Comments

• The OS sets this port to 7Ch, which results in video memory being accessible at FC00-FFFFh. However, it only sets it when the CPU is reset, so it should be set back to 7Ch (or 3Ch, BCh, or FCh, which behave identically) before returning to the OS.
• Changing this port causes the LCD image to change at the start (probably) of the next frame, so it can be used for double buffering, but not for a split screen effect.