pcjs/devices/pc8080/rom/vt100

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DEC VT100 ROMs

8080 Firmware (8Kb)

The DEC VT100 Terminal used four 2Kb ROMs to store all the firmware used by the 8080 processor. The combined contents of those ROMs have been stored as an 8Kb JSON image in VT100.json.

As Trammell Hudson's VT100 Page explains:

There are four 8316E ROM chips for the 8080 CPU on the logic board. Since they are custom mask ROMs,
Digital was able to have a different chip-select bit pattern and avoided having separate NOT gates on
the inputs.  Note that the schematic appears to be incorrect -- E56 is the low order, and E40 is the
high order address bits.

He then describes the memory map as follows:

ROM chip    CS1 (20)    A11/CS2 (18)    A12/CS3 (21)    Mapped address
E40         H           H               H               0x1800
E45         H           L               H               0x1000
E52         H           H               L               0x0800
E56         H           L               L               0x0000

The above PCB chip locations correspond to the following DEC ROMs:

• E56: 23-061E2.bin
• E52: 23-032E2.bin
• E45: 23-033E2.bin
• E40: 23-034E2.bin

And sure enough, concatenating those four DEC ROMs:

cat 23-061E2.bin 23-032E2.bin 23-033E2.bin 23-034E2.bin > VT100.bin

produces a perfect match for Trammell Hudson's VT100.bin.

Character Generator (2Kb)

The VT100 also used one 2Kb character generator ROM, which is stored in 23-018E2.json. The ROM contains 128 rows of character data, 16 bytes per character. More on the format of that data later.

Disassembling the 8080 Firmware

Following in the footsteps of vt100romhax, I disassembled the ROM, using dz80 from D52 (here's the manual):

dz80 -80 VT100.bin

This produced VT100.d80, which I renamed to VT100.asm. I fixed one dz80 bug, replacing references to X2000 with 2000h, and then hand-merged most of the comments from haxrom.d80. This required some selectivity, because I didn't want to inadvertently include any of phooky's screensaver-related mods to the ROM.

Finally, I verified that reassembling VT100.asm with asm8080 produced the original VT100.bin; after adding the correct number of nop instructions to the end to the source file, the binaries matched.

The other advantage of reassembling the code is that the resulting VT100.txt makes it easy to export comments and other symbolic information to VT100.map, which can then be included in the VT100.json ROM dump and passed on to the PC8080 Debugger. Here are the rebuild steps:

asm8080 -lVT100.txt VT100.asm
grep -E "[0-9]+ [0-9A-D]+.*;;" VT100.txt | sed -E "s/ *[0-9]+ ([0-9A-F]+).*;(;.*)/     \1   .   \2/" > VT100.map
filedump --file=VT100.bin --format=bytes --output=VT100.json --comments --overwrite

You can omit --comments from the filedump command to reduce the size of the VT100.json file.

Some VT100.asm clean-up remains, because there are still chunks of data that were incorrectly disassembled as code. From a reassembly standpoint, it doesn't matter too much, because all the 8080 instructions unambiguously reassemble into their original binary patterns, but from a readability standpoint, it's a nuisance.