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====== NEC PC-9801 - Text mode ======

===== Screen layout =====

By default, video memory is stored at ''A000h:offset'' for characters and ''A200h:offset'' for attributes - both are stored as 16-bit words.

===== Attribute bits =====

  * bit 0 - rendered (1), hidden (0)
  * bit 1 - blink (1)
  * bit 2 - reverse video (1)
  * bit 3 - underline (1)
  * bit 4 - VL/BG ???
  * bit 5 - blue (1)
  * bit 6 - red (1)
  * bit 7 - green (1)

Bits 5-7 will function as effective 8-level grayscale on monochrome displays, with values 0 through 7 respectively providing brighter grayscale levels.

===== Custom glyphs =====

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 16x16 character data. TODO