---
layout: page
title: VT100 Terminal
permalink: /devices/pc8080/machine/vt100/
machines:
- type: pc8080
id: vt100
debugger: true
---
VT100 Terminal
--------------
This is where we'll be testing another 8080-based machine: the VT100 Terminal. Unlike other VT100 emulations,
this will simulate a VT100 by running the terminal's original firmware inside the [PC8080](/modules/pc8080/) CPU emulator.
{% include machine.html id="vt100" %}
VT100 Memory Usage
------------------
As described in the [Technical Manual (July 1982)](http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/terminal/vt100/EK-VT100-TM-003_VT100_Technical_Manual_Jul82.pdf),
p. 4-15, 8Kb (0x2000) of ROM is located at 0x0000, and 3Kb (0x0C00) of RAM immediately follows it at 0x2000. The ROM at
0x0000 contains all the VT100's 8080 code. The VT100 also contains a 2Kb character generator ROM, but that ROM is not
addressable by the CPU; it is used directly by the Video Processor.
[vt100romhax](http://vt100romhax.tumblr.com/post/90697428973/the-vt100-memory-map-and-8080-disassembly)
(aka [phooky](https://github.com/phooky)) further explains VT100 memory usage:
Start End Size Description
0x0000 0x1fff 8K Basic ROM
0x2000 0x2012 18B Blank lines for refresh (6 x 3B)
0x2012 0x204f 61B Stack area (grows down from 0x204e)
0x204f 0x22d0 641B Scratch Pad/Setup Area(?)
0x22d0 0x2c00 2352B Screen RAM
VT100 I/O Ports
---------------
From p. 4-17 of the Technical Manual:
READ OR WRITE
00H PUSART data bus
01H PUSART command port
WRITE ONLY (Decoded with I/O WR L)
02H Baud rate generator
42H Brightness D/A latch
62H NVR latch
82H Keyboard UART data input
A2H Video processor DC012
C2H Video processor DC011
E2H Graphics port
READ ONLY (Decoded with I/O RD L)
22H Modem buffer
42H Flags buffer
82H Keyboard UART data output
The PC8080 ChipSet component deals with the ER1400's Non-volatile RAM (NVR) ports, while the Keyboard component
deals with the Keyboard UART ports.
VT100 Video Processor
---------------------
Normally, the PC8080 Video component allocates its own video buffer, based on the specified buffer address
(*bufferAddr*) and other dimensions (eg, *bufferCols* and *bufferRows*), but the VT100 is a little unusual:
it has a custom Video Processor that uses DMA to request character data from any region of RAM, one line at a time.
It always defaults to address 0x2000 for the first line of character data, but each line terminates with 3 bytes
containing the attributes and address of the next line, so the location of subsequent lines will vary, depending
on the type of line:
- Single-wide characters (80 or 132 columns)
- Double-wide characters (40 or 66 columns)
In addition to single-wide vs. double-wide, line attributes can also specify double-high, along with whether the
top or bottom halves of the double-high characters should be displayed, because double-high always implies double-wide
(ie, there is no support for double-high, single-wide characters).
Conssequently, a VT100 [machine XML file](machine.xml) must set the Video component's *bufferRAM* property
to "true", indicating that existing RAM should be used, and a new property, *bufferFormat* must be set to "vt100",
enabling support for the VT100's line data format; eg: