Updated the page on DEC V100 ROMs to include more information on the character generator ROM
This commit is contained in:
Jeff Parsons
Jeff Parsons
parent
47b12390d0
commit
15e97df68e
1 changed files with 61 additions and 1 deletions
|
|
@ -43,7 +43,67 @@ produces a perfect match for Trammell Hudson's [VT100.bin](http://trmm.net/image
|
|||
### Character Generator (2Kb)
|
||||
|
||||
The VT100 also used one 2Kb character generator ROM, which is stored in [23-018E2.json](23-018E2.json).
|
||||
The ROM contains 128 rows of character data, 16 bytes per character. More on the format of that data later.
|
||||
The ROM contains 128 rows of character data, 16 bytes per character. Only 10 of each of the 16 bytes are used.
|
||||
|
||||
To understand the format of the character data, let's take a look at the data for the letter "A", which has an ASCII code
|
||||
of 65 (0x41). That means the data for "A" should start at offset 0x41 * 0x10, or 0x410. Here's the corresponding line from
|
||||
[23-018E2.json](23-018E2.json):
|
||||
|
||||
0x10,0x28,0x44,0x82,0xFE,0x82,0x82,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00, // 0x00000410 .(D.............
|
||||
|
||||
and if we display the data as bits instead of bytes:
|
||||
|
||||
. . . 1 . . . . // 0x10
|
||||
. . 1 . 1 . . . // 0x28
|
||||
. 1 . . . 1 . . // 0x44
|
||||
1 . . . . . 1 . // 0x82
|
||||
1 1 1 1 1 1 1 . // 0xFE
|
||||
1 . . . . . 1 . // 0x82
|
||||
1 . . . . . 1 . // 0x82
|
||||
. . . . . . . . // 0x00
|
||||
. . . . . . . . // 0x00
|
||||
. . . . . . . . // 0x00
|
||||
|
||||
it should be clear how the shape of the character is defined. However, the character generator ROM doesn't tell the whole story,
|
||||
because the VT100's display circuitry has a few additional tricks up its sleeve.
|
||||
|
||||
Since the ROM contains only 8 bits of data for each character row, and character cells are effectively 10x10 (or 9x10 in 132-column
|
||||
mode), there is a natural gap between adjacent characters. However, when horizontal line-drawing characters are used, the VT100 wants
|
||||
those characters to seamlessly connect with one another, so if the final bit in any row of character data is set, the VT100 will
|
||||
"replicate" that bit across the rest of cell.
|
||||
|
||||
As page 4-52 of DEC's [VT100 Technical Manual (July 1982)](http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/terminal/vt100/EK-VT100-TM-003_VT100_Technical_Manual_Jul82.pdf)
|
||||
explains:
|
||||
|
||||
Data, coming either from the screen RAM for scan 1 or the line buffer for scans 2 through 10, becomes part of an
|
||||
address to a character generator ROM. (See Figure 4-6-4, Character Generator Example.) The rest of the address comes
|
||||
from a scan counter in the DC012 control chip. The scan counter addresses the ROM according to which oftne ten scans
|
||||
is to be displayed. The 4-bit scan counter skips over the other 6 possible addresses to the ROM, so the ROM contains
|
||||
data in only 10 out of 16 locations. The output of the ROM is eight bits that represent the pattern of sequential
|
||||
dots to be displayed for that character on that scan. The eight bits enter the video shift register, a serializer
|
||||
that converts the eiight parallel bits into a one-bit-wide stream. An extra flip-flop stores the last bit so it can
|
||||
be output to the stream two or three extra times (depending on line length) to fill the intercharacter space.
|
||||
|
||||
Also, VT100 circuitry includes a "Dot Stretcher" to make vertical lines appear as thick as horizontal lines. If you look
|
||||
at page 4-76 of DEC's [VT100 Technical Manual (July 1982)](http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/terminal/vt100/EK-VT100-TM-003_VT100_Technical_Manual_Jul82.pdf),
|
||||
you'll see the effect of the VT100's Dot Stretcher. Here's a copy of DEC's illustration:
|
||||
|
||||
20 dots for 2 characters ("Ap") in 80-col mode Actual characters displayed after dot stretching
|
||||
|
||||
0 . . . . . . . . . . . . . . . . . . . . 0 . . . . . . . . . . . . . . . . . . . .
|
||||
1 . . . . 1 . . . . . . . . . . . . . . . 1 . . . . 1 1 . . . . . . . . . . . . . .
|
||||
2 . . . 1 . 1 . . . . . . . . . . . . . . 2 . . . 1 1 1 1 . . . . . . . . . . . . .
|
||||
3 . . 1 . . . 1 . . . . 1 . 1 1 1 1 . . . 3 . . 1 1 . . 1 1 . . . 1 1 1 1 1 1 1 . .
|
||||
4 . 1 . . . . . 1 . . . 1 1 . . . . 1 . . 4 . 1 1 . . . . 1 1 . . 1 1 1 . . . 1 1 .
|
||||
5 . 1 1 1 1 1 1 1 . . . 1 1 . . . . 1 . . 5 . 1 1 1 1 1 1 1 1 . . 1 1 1 . . . 1 1 .
|
||||
6 . 1 . . . . . 1 . . . 1 . 1 1 1 1 . . . 6 . 1 1 . . . . 1 1 . . 1 1 1 1 1 1 1 . .
|
||||
7 . 1 . . . . . 1 . . . 1 . . . . . . . . 7 . 1 1 . . . . 1 1 . . 1 1 . . . . . . .
|
||||
8 . . . . . . . . . . . 1 . . . . . . . . 8 . . . . . . . . . . . 1 1 . . . . . . .
|
||||
9 . . . . . . . . . . . 1 . . . . . . . . 9 . . . . . . . . . . . 1 1 . . . . . . .
|
||||
|
||||
The PC8080 [Video](/modules/pc8080/lib/video.js) function *createFontVariation()* takes both the "dot replication" and "dot stretching"
|
||||
features into account (along with other parameters, such as underlining and reverse video) when converting the character generator ROM data
|
||||
into fonts.
|
||||
|
||||
### Disassembling the 8080 Firmware
|
||||
|
||||
|
|
|
|||
Loading…
Reference in a new issue