Specifications

CONSTRUCTION
The character generator is just a list
of dots for the characters in an order. It
does not necessarily require any hard-
ware. Though such a list of dots, if stored
in this order in an EPROM, can be a hard-
ware component. In the PC based design,
this is just a file containing the dot pat-
terns, while in the design using a dedi-
cated CRT controller with a microproces-
sor, this is actually a hardware compo-
nent, i.e. an EPROM.
Addressing mode for character gen-
erator file scheme. Let us take letter ‘d’
in Hindi, which has the ASCII-equivalent
code of 64 hex. We need a high address
and a low address as usual. Supposing
the high address for Hindi starts at page
10 and it goes up to page17 (with each
page comprising 256 bytes). In page 10,
locations 00 through 7F are used for stor-
ing the low addresses while 80 through
FF are used for storing high addresses
for each character. Accordingly for ‘d’, the
low address is stored at 10 64 and the
high address at 10 E4 (1064 + 80 = 10E4).
If we have a look at the hex contents of
these two locations, we shall find:
Address Data Comments
10 64 B4 LS Byte of Address
10 E4 04 MS Byte of Address
Note: The page/location-wise hex
contents of file containing these indi-
rect addresses and dot codes are pro-
posed to be issued in EFY-CD during
Sept. 2000.
It means that the actual code is start-
ing from the address 14 B4 (1000 + 04B4).
So, this is indirect addressing mode.
The actual hex values of the dots for
each of the twelve lines (in 18 bytes) for
‘d’ are stored at consecutive locations,
starting with address 14B4, as stated ear-
lier.
This indirect addressing scheme is
used because it enables us to use differ-
ent types of fonts later, by just pointing
to a different address table. Also, the ad-
dress table corresponds to the ASCII code,
Fig. 1(b): 6845 character and video generator portion of multilingual display system
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