The WE control code tells the OSD generator that the character codes following belong to another displayed window at the next

window location. A WE control code may follow normal characters or an SL parameter, but never an RE control code.

WRITING TO THE PAGE RAM

The Display Page RAM can contain up to 512 of the above listed characters and control codes. Each character, or control code

will consume one of the possible 512 locations. For convenience, a single write instruction to bit 3 of the Frame Control Register

(0x8400) can reset the page RAM value to all zero.

Display Window 1 will also start at the first location (corresponding to the I

the Skip-Line (SL) code associated with the first row of Display Window 1. Subsequent locations should contain the characters

to be displayed on row 1 of Display Window 1, until the RE character code or WE character code is written into the Display

Page-RAM.

The skip-line parameters associated with the next row must always be written to the location immediately after the preceding

row’s row-end character. The only exception to this rule is when a window-end character (value 0x0000) is encountered. It is

important to note that a row-end character should not precede a window-end character (otherwise the window-end character will

be interpreted as the next row’s skip-line code). Instead, the window-end character will both end the row and the window making

it unnecessary to precede it with a row-end character.

The I

attribute code to send in a string as follows:

Byte #1—I

Byte #2 — LSB Register Address

Byte #3 — MSB Register Address

Byte #4 — Attribute Table Entry to use for the following characters

Byte #5 — First display character, SL parameter, RE or WE control code

Byte #6 — Second display character, SL parameter, RE or WE control code

Byte #7 — Third display character, SL parameter, RE or WE control code

Byte #n — Last display character in this color sequence, SL parameter, RE or WE control code to use the associated Attribute

Table Entry.

The Attribute Table Entry (Byte #4, of the above) is automatically associated with each subsequent display character or SL code

written. The following are examples of how the Display Page RAM associates to the actual On-Screen Display Window #1.

EXAMPLE 1:

A 3x3 character matrix of yellow characters on a black background is to be displayed on the screen of all the same color, using

2-color character codes:

The actual On-Screen Display of Window #1 is shown in Figure 21. Note the dotted white lines are not actually part of the OSD

image to be displayed. They are shown here only to designate character boundaries.

Notes:

•

Every row must begin with an attribute and an SL. Display Page RAM memory location 0x8000 will always be associated with

the SL of row 0 of Display Window #1.

•

Every row except the last row of a Display Window must end with an RE character. The character immediately after an RE

character is always the SL value for the next row.

•

The last row in a Display Window must be a WE character. The WE character must NOT be preceded by an RE character.

•

The entire Display Window may be written in a single I

palette) does not change for the entire Display Window.

•

The Attribute Table Entry that is associated with RE and WE characters are “don’t cares”. So in general it is most efficient just

to allow them to be the same value as the Attribute Table Entry associated with the previous display character.

20023432

FIGURE 21. Example 1 OSD

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