DEC VT220 - Page 106

• Alternate character generator RAM contains "soft"
character set (down-line loadable characters), with font
storage of up to 94 characters (each character consisting
of 10 bytes of data, one byte r each scan line).
• Character generator ROM -- contains "hard" character set.
Which character generator is selected for character data output
(CHR0 H-CHR7 H) is determined by the ALT BEL H signal from the
access mux, with ALT SEL H developed either from CPU logic input
(during CPU logic read/write access of alternate character
generator) or from attribute circuits input developed from bit 5
of the attribute associated with character to be processed.
Character latch 2 provides the base address for characters being
accessed from either of the two character generators, while the
address mux provides the scan line address.
The character ROM contains two
data, with each page containing
bits wide (which page is to be
input) •
entical es of character format
256 rows of data, each data row 16
accessed is defined by CHARG SEL H
Figure 6-18 provides a general map of
in this figure, 288 characters are
data rows: 256 characters, packed
racter ROM. As shown
eked into each page of 256
one per data row, and 32
DATA DATA DATA DATA
ROW BYTES BYTES BYTES
ADDRESS
(}-9 A+B C-F
~ rr---_A ........ __ ~,~ ~~
100X Hi060x H l!-, __ S;:..P_A.;.,;CE:;.,:I.;..CR_M_3_2.:...) --1 CRM ! FilL
101 X HiOO1 X H ,; SPECIAL GRAPHICS I 10) i CHARACTERS
I '
11 EX H/01 FX H r SPECIAL GRAPHICS ;;1'.1 1FILL
120X H/020X H!REVERSE QUESTION (CRM 25) 17) ICHARACTERS
121X H/021X HI... ASCII _ J
y ~ ~
17EX H/07EX HI-- AS_C_il __ --; CRM FILL
17FX
H!07FX H CRM (127-160, I (25) 1CHARACTERS
I '
1AOX H/OAOX H !-- __ CR_M_I_l 2_7-_1_60_1_---1
lA1X H/OA1X H DEC
SUPPLEMENTAL
FilL
CHARACTERS
1 FOX H/DF OX H
1
EEX H/OFEX H
1
FFX HIOFFX H
-
DEC
SUPPLEMENTAL
CRM 12541
CHARACTERS
CRM
(255)
Figure 6-18 Character ROM Map: Overview
6-22