By default, video memory is stored at A000h:offset for characters and A200h:offset for attributes - both are stored as 16-bit words.
Bits 5-7 will function as effective 8-level grayscale on monochrome displays, with values 0 through 7 respectively providing brighter grayscale levels.
The PC-9801 provides a facility to upload a limited amount of custom glyphs for use by the graphics chip. These are provided as 34-byte glyphs, which constitute 2 bytes of header and 32-byte 16×16 character data. TODO