Screen registers

Owned by Gunther Strube
Last updated: May 06, 2015 by Elpekos
3 min read

Screen registers

The addresses of the screen files are held in BLINK registers PB0 to PB3 and SBR. These registers tell the blink where are the screen files in physical memory. Thus the blink can generate the data stream sent to the LCD. These registers are write-only and are more than 8 bits wide. Their softcopies are stored at ($046x) for the most significant bits and ($047x) for the 8 right bits. They are set by OS_Sci call which is using OUT (C),A instructions. B always contains the left bits of data (more than 8), A the data, C the port. So, A is ($047x) and B is ($046x). Please, refer to the Screen Files chapter for more about their structure.Their width determine the file granularity in a bank. For example, the LORES1 can be stored at 0, 4, 8 or 12K.  The role of these registers is as follows:

Register  Name   Role   Length     Granularity  Width    Softcopies ($046x)       ($047x)
PB0 ($70) SC_LR0 LORES0 512 bytes  512 bytes    13 bits             5 bits right  8 bits right
PB1 ($71) SC_LR1 LORES1 3.5K bytes 4K bytes     10 bits             2 bits right  8 bits right
PB2 ($72) SC_HR0 HIRES0 6K bytes   8K bytes     9 bits              1 bit right   8 bits right
PB3 ($73) SC_HR1 HIRES1 2K bytes   2K bytes     11 bits             3 bits right  8 bits right
SBR ($74) SC_SBR SBF    2K bytes   2K bytes     11 bits             3 bits right  8 bits right


How to generate address from blink screen register softcopies (iner of the OS_SCI call)

h : bit from $046x
l : bit from $047x
                          Insert 00 bits here
                          ||
                          vv
PB0 : 13 bits : hhhh hlll 00ll lll0
PB1 : 10 bits : hhll llll 00ll 0000
PB2 :  9 bits : hlll llll 00l0 0000
PB3 : 11 bits : hhhl llll 00ll l000
SBR : 11 bits : hhhl llll 00ll l000

 

Example, on an Expanded machine :

Register : ($046x) ($047x) -> Offset
  PB0      $04     $31     -> $212200
  PB1      $00     $1C     -> $070000
  PB2      $00     $43     -> $212000
  PB3      $00     $39     -> $070400
  SBR      $01     $0F     -> $213800

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