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Part SEVEN: Explanations about the video display
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The TEXT mode is used by default when you boot (!!!) the computer. But it can be called with the TEXT
command in Basic. The modes LORES 0 and LORES 1 are some variations of TEXT. From a memory point of view, TEXT mode requires 1120 bytes for display (28 lines of 40 rows), and 1920 bytes for storing the character set. This makes a total of 3040 bytes. |
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The HIRES mode can be called by using the HIRES command in Basic. In HIRES, we always have these 1920 bytes for the characters, but the display takes much more: 8000 bytes for the graphics part (200 lines of 40 bytes), and 120 bytes for the 3 text lines under the graphic screen. This makes a total of 10208 bytes. |
An ORIC always use a monochrome display. The fact you can use 8 colors at the same times on the screen doesn't mean that you're not in monochrome.
I explain.
For the video chip (ULA: Undedicated Logic Array) or the Oric, a pixel is either a foreground pixel, or a background pixel. When you draw a line, a circle, a dot, put a letter on the screen, etc... you never have the opportunity to specify it's color. The only effect is that some pixels of the screen switch between the state OFF (meaning it's a background pixel) and the state ON (it's a foreground pixel).
By default, the colors on the screen are WHITE INK and BLACK PAPER. The Oric is able to display up to 8 colors at the same time, so here is how it works.
But before of that, I've to explain how the video memory works.
In TEXT mode, you can see the screen like a giant (!) array of 40 rows by 28 lines.
In general, the first line is reserved by the system, and is used for displaying infos like the file beeing loaded, the status of the CapsLock trigger, and stuff like that. It is called the status line.
In this array, you can put some values. If these values (bytes) are in the range 32-127, it means this is a character to display. It is standard ASCII codes. I said standard. Values above 127 are not ASCII values.
If the value is above 127, it means that the character is in Video Inverse. In that case, you substract 128, and you have the standard ASCII value, but displayed with different colors. As I'm talking about colors, here is a little table that shows the available colors, and the resulting inverted colors:
| NUMBER | STANDARD COLOR | INVERTED COLOR |
| 0 | BLACK | WHITE |
| 1 | RED | CYAN |
| 2 | GREEN | MAGENTA |
| 3 | YELLOW | BLUE |
| 4 | BLUE | YELLOW |
| 5 | MAGENTA | GREEN |
| 6 | CYAN | RED |
| 7 | WHITE | BLACK |
As you can see, the video inversion consist in an inversion of each component of the color.
Now, about the colors, you have to understand the principle of line attributes.
When you want to change a color, you have to insert an attribute that will say to the ULA, that now the colors are not WHITE or BLACK, but something else. An attribute is any value find by the ULA in the screen, with a value less than 32. Almost every attribute start it's effect at the position he was read, and this until another attribute changes something, or until we reach the end of the current video line. When a new line starts, the ULA resets all its internal attribute registers, set the background color (the paper) to BLACK and the foreground color (the ink) to WHITE.
This simplicity is cool. But the drawback is that when you put an attribute somewhere, you cannot put something else.
And this something else could have been character or graphics.
Before I continue, I now gives you the list of all available attributes:
| VALUE | EFFECT |
| 0 | Change INK to BLACK |
| 1 | Change INK to RED |
| 2 | Change INK to GREEN |
| 3 | Change INK to YELLOW |
| 4 | Change INK to BLUE |
| 5 | Change INK to MAGENTA |
| 6 | Change INK to CYAN |
| 7 | Change INK to WHITE |
Paper (background) attributes
| VALUE | EFFECT |
| 16 | Change PAPER to BLACK |
| 17 | Change PAPER to RED |
| 18 | Change PAPER to GREEN |
| 19 | Change PAPER to YELLOW |
| 20 | Change PAPER to BLUE |
| 21 | Change PAPER to MAGENTA |
| 22 | Change PAPER to CYAN |
| 23 | Change PAPER to WHITE |
CharSet modifier attributes
| VALUE | EFFECT |
| 8 | SingleSize NonBlinking Standard Charset |
| 9 | SingleSize NonBlinking Alternate Charset |
| 10 | DoubleSize NonBlinking Standard Charset |
| 11 | DoubleSize NonBlinking Alternate Charset |
| 12 | SingleSize Blinking Standard Charset |
| 13 | SingleSize Blinking Alternate Charset |
| 15 | DoubleSize Blinking Standard Charset |
| 15 | DoubleSize Blinking Alternate Charset |
Video control attributes
| VALUE | EFFECT |
| 24 | 60 Hz TEXT display |
| 25 | 60 Hz TEXT display |
| 26 | 50 Hz TEXT display |
| 27 | 50 Hz TEXT display |
| 28 | 60 Hz HIRES display |
| 29 | 60 Hz HIRES display |
| 30 | 50 Hz HIRES display |
| 31 | 50 Hz HIRES display |
With all these tables, you can now understand why it is impossible to access to the 2 first columns of the screen.
When you type PAPER 4, the first column of the screen is filled with the attribute 20, and when you type INK 6, the second column is filled with the attribute 6. If you try to poke something in these adresses, the text at the right is altered (meaning that the text become WHITE, or the background become BLACK).
For aesthetical reasons, I give you an advice: If you want to display text with colors, better try to put the INK attribute first, and then the PAPER attribute. The result is the same, except that the first column is black. So it look like if only ONE column where unusable. It's the same thing in HIRES, it reduces the shocking effect of graphics starting very far from the left of the screen.
It's possible to enable special effects for the display of text (and for graphics too with the blinking).
Attributes 8 to 15 changes the way the text is displayed at screen. You can select the alternate charset, display the text in double height mode, and at last, made the text blinking.
All theses specials functions can be used with only ONE attribute, so it don't take to much space on screen.
The Oric is able to use 2 different character-set for displaying text. The normal charset is copied from the ROM at the startup, while the second one is generated by a ROM routine. The second charset looks like a 2x3 blocs matrix. Each of these pixels is about 3x3 pixels, except the middle line that is only 3x2 pixels (because 3 blocs of 3 pixels makes 9 pixels ---> more than the 6x8 character size).
Note: most computers uses an 8x8 font size, but in the Oric, it is only 6x8. This explains the strange HIRES resolution. 240 pixels wide looks like 40x6 pixels. And as the video chip process HIRES a bit like TEXT, it's normal that the constraints looks the same. For the ULA a byte can contain only 6 displayable bits, because the 8th bit represents video inversions, and the 7th bit is used as an "attribute active" flag.
This charset can be useful for displaying pseudo-graphics since it's easy to compute the appearance of a block. The following table gives the values to use for activate one of the blocks: (Always add 32 to this value)
