RAM Data Format (Standard LM1247 Mode)

Each of the 512 locations in the page RAM is comprised of a 13-bit code consisting of an 8-bit character or control code, and a

4-bit attribute code. Each of the characters is stored in sequence in the page RAM in bits 7:0. Special codes are used between

lines to show where one line ends and the next begins, and also to allow blank (or ‘skipped’) single scan lines to be added

between character lines. Table 8. Page RAM Format (Standard Mode) shows the format of a character stored in RAM. Note that

even though this is a 13- bit format, reading and writing characters and codes is done in 8-bit bytes.

TABLE 8. Page RAM Format (Standard Mode)

ATTRIBUTE CODE

CHARACTER CODE

CC[8]

BANK SEL.

BANK CHARACTER

ATT[3:0]

0

CC[7:6]

CC[5:0]

Bits 7–6 determine which Bank Select Register is used to look up the 3-bit address of the bank where the character will be called

from. Bits 5–0 determine which of the 64 characters is called from that bank. Bit 8 is unused since this would be the 9th character

code bit for the mode below, however in this mode, the character code is defined with only 8 bits. Bits 12–9 address one of the

16 attributes in the table containing the colors and enhanced features to be used for this particular character. Two separate

attribute tables are used, one for 2-color characters, and the other for 4-color characters. Note there are 16 available attributes

for 2-color characters and a different set of 16 available attributes for 4-color characters. It is the bank number in the register

called by the Bank Select bits, which determines whether the character has a 2-color or 4-color attribute.

RAM Data Format (9-Bit Character Code Definition Mode)

Each of the 512 locations in the page RAM is comprised of a 13-bit code consisting of a 9-bit character or control code, and a

4-bit attribute code. Each of the characters is stored in sequence in the page RAM in bits 8:0. Special codes are used between

lines to show where one line ends and the next begins, and also to allow blank (or ’skipped’) single scan lines to be added

between character lines. Table 9. Page RAM Format (9-bit mode) shows the format of a character stored in RAM. Note that even

though this is a 13- bit format, reading and writing characters and codes is done in 8-bit bytes.

TABLE 9. Page RAM Format (9-bit mode)

ATTRIBUTE CODE

CHARACTER CODE

ATT[3:0]

CC[8:0]

Bits 8–0 determined which of the 512 characters is to be called from the character ROM. Bits 12–9 address one of the 16

attributes in the table containing the colors and enhanced features to be used for this particular character. Two separate attribute

tables are used, one for 2-color characters, and the other for 4-color characters. Note there are 16 available attributes for 2-color

characters and a different set of 16 available attributes for 4-color characters.

End-Of-Line Code

To signify the end of a line of characters, a special End-of-Line (EOL) code is used in place of a character code. This code, shown

in Table 10. End-Of-Line Code tells the OSD generator that the character and attribute codes which follow must be placed on a

new line in the displayed window. Bits 8–1 are zeros, bit 0 is a one. The attribute that is stored in Page RAM along with this code

is not used.

TABLE 10. End-Of-Line Code

ATTRIBUTE CODE

END-OF-LINE CODE

ATT[3:0]

000000001

Skip-Line Code

In order to allow finer control of the vertical spacing of character lines, each displayed line of characters may have up to 15

skipped (i.e., blank) lines between it and the line beneath it. Each skipped line is treated as a single character pixel line, so

multiple scan lines may actually be displayed in order to maintain accurate size relative to the character cell. An internal algorithm

maintains vertical height proportionality (see the section on Constant Character Height Mechanism). To specify the number of

skipped lines, the first character in each new line of characters is interpreted differently than the others in the line. Its data are

interpreted as shown in Table 11. Skipped-Line Code, with the attribute bits setting the color of the skipped lines.

TABLE 11. Skipped-Line Code

ATTRIBUTE CODE

NUMBER OF SKIPPED LINES

ATT[3:0]

XXXXX

SL[3:0]

Bits 8–4 are reserved and should be set to zero. Bits 3–0 determine how many blank pixel lines will be inserted between the

present line of display characters and the next. A range of 0–15 may be selected. Bits 12–9 determine which attribute the pixels

in the skipped lines will have, which is always called from the two-color attribute table. The pixels will have the background color

(Color 0) of the selected attribute table entry.

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