1.1 --- a/ULA.txt Sun Aug 23 17:26:06 2020 +0200
1.2 +++ b/ULA.txt Sun Aug 23 22:26:14 2020 +0200
1.3 @@ -733,19 +733,31 @@
1.4
1.5 From these, the following properties emerge:
1.6
1.7 - * The number of bytes per character row
1.8 - * The size of the entire display in bytes
1.9 - * Pixel frequency or horizontal resolution
1.10 - * The number of character rows
1.11 + Property Influences
1.12 + -------- ----------
1.13 + Character row size (bytes) Retrieval rate
1.14 +
1.15 + Number of character rows Text mode setting
1.16 +
1.17 + Display size (bytes) Retrieval rate (character row size)
1.18 + Text mode setting (number of rows)
1.19 +
1.20 + Pixel frequency Retrieval rate
1.21 + Horizontal resolution (pixels) Colour depth
1.22
1.23 One can imagine a register bitfield arrangement as follows:
1.24
1.25 - * Pixel depth: log2(depth)
1.26 - (00 - 1 bit per pixel, 01 - 2 bits per pixel, 10 - 4 bits per pixel)
1.27 - * Retrieval rate: 2 - fetches per cycle pair
1.28 - (0 - twice, 1 - once)
1.29 - * Text mode enable
1.30 - (0 - disable, 1 - enable)
1.31 + Field Values Formula
1.32 + ----- ------ -------
1.33 + Pixel depth 00: 1 bit per pixel log2(depth)
1.34 + 01: 2 bits per pixel
1.35 + 10: 4 bits per pixel
1.36 +
1.37 + Retrieval rate 0: twice 2 - fetches per cycle pair
1.38 + 1: once
1.39 +
1.40 + Text mode enable 0: disable/off text mode enabled
1.41 + 1: enable/on
1.42
1.43 This arrangement would require four bits. However, one bit in &FE07 is
1.44 seemingly inactive and might possibly be reallocated.