EF9345 datasheet(29/38 Pages) STMICROELECTRONICS

29 / 38 page

80 CHAR/ROW CHARACTER CODES

To display pages in 80 character per row format,

one of two character code formats must be se-

lected :

- Long (12 bits) code : 4 parallel attributes and large

on-chip 1024 semigraphic character set,

- Short (8 bits) code : no attribute, no semigraphic

set.

Both formats address the on-chip G0 set (128

characters 6 x 10). None allows UDS addressing.

Long Codes

Each 6 pixels x 10 lines character window on the

screen is associated with a 12-bit code in memory,

namely a C byte and an attribute nibble A (Fig-

ure 18). C7 bit designates the set.

- Alphanumeric set : C7 = 0

C(0:6) designates one out of 128 alphanumeric

characters in the G0 on-chip set. This set is

common to the 40 char/row format, with the 2 right

most columns truncated (see Figure 34).

A(0:3) gives 4 parallel attributes.

- Mosaïc set : C7 = 1

A(1:3) and C(0:6) address a dedicated mosaïc

character. Each of these address bits controls the

foreground/background status of a 3 pixels x 2

lines sub-window : foreground when the bit is set.

A0 provides a color select attribute.

Short Codes

They are derived from the long code by giving a 0

implicit value to each bit of the A nibble : positive,

not underlined, not flashing.

Packing the Codes in Memory

Long codes are paired. A pair is packed in a 3-byte

word. Therefore, the 80 codes of a row fill a 120-

byte row buffer (Figure 29). The left most position

on the screen is even. Its corresponding C byte is

at the beginningof the first buffer. The next position

on the screen is odd. Its corresponding C byte is at

the beginning of the second buffer. Both nibbles

are packed in the third buffer. With short codes, the

same scheme yields 80-byte row buffers.

Access to the Codes in Memory

KRL command transfers 12 bits from/to the R1 and

R3 registers to/from memory. The read modify

write operation, necessary to write the A nibble in

memory, is automatically performed provided that

the A nibble is repeated in the R3 register (Fig-

ure 30). Dedicated auto-incrementationis also per-

formed when required.

KRC command does a similar job for the short

codes (Figure 31).

A very simple scheme allows the microprocessor

to transcode an horizontal screen location into a

pointer (Figure 32). The joint use of this scheme

with the dedicated command alleviates all the

packing/unpacking troubles.

765432

XXXXXX

76543210

XXXXX

ALPHANUMERIC CHAR CODE

N = Negative

F = Flash

U = Underline

D = Color set

128 ALPHANUMERICS

In G

0 set.

MOSAIC CHAR CODE

3 pels

DEDICATED

MOSAIC SET

Figure 28 : 80 Char/Row Character Code

EF9345

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