4.3 Video-RAMaccess from the ROM-monitor

Addressing the Video-RAM from the ROM-monitor is also done with bankswitching. The screen does change when you do a bankswitch, because we jump from the 700-mode to an 800-mode, which alters the screen resolution. This is not the case when you do this from BASIC, because the computer will already be in 800-mode.

In chapter 3 you have seen two programs that do something with the Video-RAM from the ROM-monitor.

In this part of chapter 4 we will look at several programs that do something with Video-RAM. The assembler listing and explanation will be given as well because this is more conveniently arranged and because the assembly chapter is large enough.

Program 1: Alphabet-scroll.

This program scrolls the alphabet upward over the screen.

6000 DB  600B 03  6016 23  6021 00  602C 77  6037 11  6042 23
6001 E0  600C D3  6017 0B  6022 00  602D 00  6038 00  6043 13
6002 3E  600D CC  6018 78  6023 00  602E 13  6039 80  6044 0B
6003 00  600E 21  6019 B1  6024 00  602F 00  603A 21  6045 78
6004 D3  600F 00  601A C2  6025 11  6030 00  603B D8  6046 B1
6005 CE  6010 80  601B 14  6026 30  6031 00  603C 7F  6047 C2
6006 3E  6011 01  601C 60  6027 5F  6032 00  603D 01  6048 40
6007 00  6012 40  601D 3E  6028 21  6033 7B  603E 40  6049 60
6008 D3  6013 1F  601E 37  6029 19  6034 32  603F 1F  604A C3
6009 F0  6014 AF  601F D3  602A 9F  6035 25  6040 1A  604B 25
600A 3E  6015 77  6020 F0  602B 1A  6036 60  6041 77  604C 60

Filename? A-Z SCROLL
Top adrs? 6000
End adrs? 604C
Exc adrs? 6000
Start this program with G6000.
This program can only be stopped by pressing the reset button.

Assembler listing of program 1:

6000 IN   A,(E0H)
6002 LD   A,00H
6004 OUT  (CEH),A
6006 LD   A,00H
600A LD   A,03H
600C OUT  (CCH),A
600E LD   HL,8000H
6011 LD   BC,1F40H
6014 ONE:
6014 XOR  A
6015 LD   (HL),A
6016 INC  HL
6017 DEC  BC
6018 LD   A,B
6019 OR   C
601A JP   NZ,ONE
601D LD   A,37H
601F OUT  (F0H),A
6021 LD   A,00H
6023 LD   A,00H
6025 TWO:
6025 LD   DE,5F30H
6028 LD   HL,9F19H
602B LD   A,(DE)
602C LD   (HL),A
602D NOP
602E INC  DE
602F NOP
6030 NOP
6031 NOP
6032 NOP
6033 LD   A,E
6034 LD   (6026H),A
6037 LD   DE,8000H
603A LD   HL,7FD8H
603D LD   BC,1F40H
6040 THREE:
6040 LD   A,(DE)
6041 LD   (HL),A
6042 INC  HL
6043 INC  DE
6044 DEC  BC
6045 LD   A,B
6046 OR   C
6047 JP   NZ,THREE
604A JP   TWO

Explanation of program 1:

First of all there is a bankswitch at address 6000.

Then, the computer is set to one of the 800-modes at addresses 6002 - 6005.

Then palette 0 with colour 0 is loaded. Palette 0 will be black. With port F0H the colour of the palette can be altered. The palette, as well as the colour will be in address A. The first number in A is the palette and the second is the colour the palette has. This all happens at addresses 6006 - 6009.

At addresses 600A - 600D the palette is selected, in this case palette 3.

From 600E - 601C the screen is cleared.

Then palette 3 is loaded with value 7 ( light-gray ) at addresses 601D - 6020.

Then a useless piece follows.

At 6025 - 6034 1/8 part of a letter is drawn at the bottom of the screen and from 6035 on, the screen is scrolled 1 line up. This cycle is repeated until the reset button is pressed.

The program also contains some empty instruction that can be recognized as the NOP instruction. The program was already written when we found out that these parts were of no use, so we replaced them with NOPs.

In the chapter about assembly we shall go into the Video-RAM even further.

Program 2: 13 rows of 16 colours.

This program draws 16 lines in 16 different colours after each other and repeats this 13 times. So this program should only work with the extra ICs. If you do not own them, you can still run this program, but you will see only 4 colours instead of 16.

You will probably understand by now why the third screen in Tut-Ench-Amun can be seen with the ICs and not without them. This is because screen 3 has more than 4 colours in use.

2000 dB  2008 D3  2010 AF  2018 20  2020 D3  2028 FB  2030 ED
2001 E0  2009 CC  2011 77  2019 21  2021 CC  2029 3C  2031 C3
2002 3E  200A 21  2012 23  201A 00  2022 06  202A FE  2032 31
2003 02  200B 00  2013 0B  201B 80  2023 28  202B 90  2033 20
2004 D3  200C 80  2014 78  201C 0E  2024 36  202C 20
2005 CE  200D 01  2015 B1  201D 0D  2025 FF  202D F2
2006 3E  200E 40  2016 C2  201E 3E  2026 23  202E 0D
2007 03  200F 1F  2017 10  201F 80  2027 10  202F 20

Filename? COLOUR ROWS
Top adrs? 2000
End adrs? 2033
Exc adrs? 2000
Start this program with G2000.
This program can only be stopped by pressing the reset button.

Assembler listing of program 2:

2000 IN   A,(E0H)
2002 LD   A,02H
2004 OUT  (CEH),A
2006 LD   A,03H
2008 OUT  (CCH),A
200A LD   HL,8000H
200D LD   BC,1F40H
2010 ONE:
2010 XOR  A
2011 LD   (HL),A
2012 INC  HL
2013 DEC  BC
2014 LD   A,B
2015 OR   C
2016 JP   NZ,ONE
2019 LD   HL,8000H
201C LD   E,0DH
201E TWO:
201E LD   A,80H
2020 THREE:
2020 OUT  (CCH,A)
2022 LD   B,28H
2024 FOUR:
2024 LD   (HL),FFH
2026 INC  HL
2027 DJNZ FOUR
2029 INC  A
202A CP   90H
202C JR   NZ,THREE
202E DEC  C
202F JR   NZ,TWO
2031 FIVE:
2031 JP   FIVE

Explanation of program 2:

At first there is a bankswitch and then we switch to the 800-mode with 16 colours.

Then the screen is ’cleared’. In fact this has no use, but it is not important. Then 16 rows with different colours are drawn 14 times.

Options of port CEH.

You have seen that with port CEH the screen resolution and the amount of colours can be altered. Now we shall discuss all functionality of port CEH. All possible values and their screen resolution will be shown. You must send these values to port CEH to get the effect.

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