EF9345 datasheet(19/38 Pages) STMICROELECTRONICS

19 / 38 page

Variable Codes

In many cases, successive characters on screen

belong to the same character set and have the

same attributes. Variable codes achieve memory

saving by storing B and A bytes only when it is

required by exploiting the C7 bit.

C7 = 1 This is a long 3-byte code.

Character set and attribute values are

completely redefined by B and A bytes.

C7 = 0 This is a short 1-byte code.

Character set and attributes value are

identical to the previous code.

A further saving comes from the fact that an accen-

tuated alphabetic character is, more often than not,

followed by a not accentuated alphabetic character.

So, G20 or G21 sets are processed as one-shot

escape with return to G0. For normal operation,

variable codes should obey the following rules :

- The first character code of any row (X = 0) should

be long.

- A character code may be short when it has the

same attributes as the previous character code

and belongs to the same set.

However :

- Any code belonging to G20 or G21 must be long

and must be repeated if the character is double

width,

- A code belonging to G0 following a G20 or G21

code may be short.

Handling the Variable Codes

During the display process, a row of variable code

should be laid in an 80/120 byte row buffer. The

first buffer holds the C bytes. The second buffer

holds the B, A file providing up to 20 long codes per

row (Figure 18). In the exceptional case when this

is not enough, the second buffer overflows in the

third one. Every code may then be long. Variable

codes can almost always achieve a memory saving

over long fixed codes and can never be worse.

The KRV command gives a very easy sequential

access to/from a row buffer from/to the microproc-

essor. This command automatically updates the C

byte and B, A file pointers (the last one when C7 is

set).

ZW, YW

BF, XF

D, Y

B, X

EXP and CMP

COMMANDS

BW (even)

BW + 1

B (even)

039

B + 1 = BF

VARIABLE

CODE

EXPANDED

CODE

Figure 17 : Expansion/Compression Move

EF9345

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