The first step in using the character map mode is to create a character set in memory (or the built-in OS character set at hex E000 may be used). The character set contains eight bytes of data for the graphics for each character. The meaning of the data depends on the mode. The character set can contain 64 or 128 characters, also depending on the mode. The MSB (Most Significant Byte) of the address of the character set is stored in CHBASE (or the OS Shadow CHBAS). Only the most significant six or seven bits of CHBAS are used (see CHBASE description in section III). The other one or two bits and the LSB of the address are assumed to be zero, so the character set must start at an acceptable page boundary.
The next step is to set up the display list for the desired mode. Then the actual display is set up. This consists of a string of character names or codes. Each name takes one byte. The last 6 or 7 bits of the name selects a character. For a 64-character set, the name would range from 0 through 63 (decimal). For a 128 character set, the range would be 0 through 127 (decimal). The upper one or two bits of the name byte are used to specify the color or other special information, depending on the mode.
Character names (codes) are fetched by the memory scan counter, and are placed in a shift register. On any given line of display the shift register rotates, changing only the name portion of the character address, as shown below.
After a full line of character data has been displayed the line counter will increment. The next line again addresses all characters by name for that line number.
In 20 character per line modes the seven most significant bits of CHBASE are used. This requires that the character set to start upon a 512 byte memory boundary. The set must contain 64 characters, 8 bytes each, giving a total of 512 bytes for the set.
The 40 character per line modes use the six most significant bits of CHBASE, forcing the character set to start on a 1K byte memory boundary.
The set must have 128 characters of 8 bytes each. This gives a total of 1024 bytes for the set.
Hex
CodeGraphics
modeChars
per lineNumber
of colorsBytes
per charChars
in setBytes in
char set2 0 40 2 8 128 1024 3 - 4 - 4 5 - 6 1 20 5 8 64 512 7 2
There are six character map modes, IR modes 2 through 7. Modes 2,6 and 7 are supported by the OS and BASIC (GRAPHICS 0,1 and 2).
In IR modes 6 and 7, the upper two bits of each character name select one of four playfield colors. For each data bit that contains a one, the selected playfield color is displayed. For each zero data bit, the background color is displayed. The four character colors plus the background color gives a total of five different colors. the mode 6 characters are eight lines high and the mode 7 characters are sixteen lines high (each data byte is displayed for two lines).
In IR modes 4 and 5, each character is only four pixels wide instead of eight (as in the other modes). Two bits per pixel of data are used to select one of three playfield colors, or background. Seven name bits are used to select the character. If the most significant name bit is a zero then data of 10 (binary) selects PFl. If the name bit 7 is one, then data bits of 10 select PF2. This makes it possible to display two characters with different colors, using the same data but different name bytes.
In IR modes 2 and 3, each pixel is half of a color clock in width. This makes it possible to have forty eight-pixel-wide characters in a standard width line. These modes are similar to memory mode F in that two luminances can be displayed, but only one color is available at a time.
In IR mode 3, each character is 10 lines high. This makes it possible to define lower case characters with descenders. The last fourth of the character set (name bits 5 and 6 equal to one) is lowered. The hardware takes the first two data bytes and moves them to the bottom of the character, displaying two blank lines at the top of the character (see next page).
In IR modes 2 and 3, bit 7 of the character name is used for inverse video or blanking. This is controlled by CHACTL (Character Control). If bit 2 of CHACTL is a one then all of the characters will be displayed upside down, regardless of mode. If CHACTL bit 1 is set, then each character which has bit 7 of its name set will be displayed in inverse video (the luminances will be reversed). If CHACTL bit 0 is set, then each character which has bit 7 set will be blanked (only background will be displayed). Characters can be blinked on and off by setting name bit 7 to 1 and toggling CHACTL bit 0. Inverse video and blank apply only to IR modes 2 and 3. If both inverse video and blank are set then the character will appear as an inverse video blank character (solid square).