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