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Update VT100 machine README

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Jeff Parsons 2016-08-03 13:57:09 -07:00 • committed by Jeff Parsons
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devices/pc8080/machine/vt100/README.md
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@ -25,7 +25,7 @@ p. 4-15, 8Kb (0x2000) of ROM is located at 0x0000, and 3Kb (0x0C00) of RAM immed
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:
(aka [phooky](https://github.com/phooky) aka Adam Mayer) further explains VT100 memory usage:
Start End Size Description
0x0000 0x1fff 8K Basic ROM
@ -57,8 +57,28 @@ From p. 4-17 of the Technical Manual:
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.
The PC8080 ChipSet component deals with the ER1400's Non-volatile RAM (NVR) ports, the Flags buffer, and the
DC011 and DC012 circuits, while the Keyboard component deals with the Keyboard UART.
You might wonder why the PC8080 Video component doesn't manage the DC011 and DC012. In fact, the above labels are misleading.
If you look at the Functional Diagram on p. 4-53 of the Technical Manual, you'll see that DC011 and DC012 are really
peripheral components providing inputs to the Video Processor. Moreover, they are not exclusive to the Video Processor.
For example, the LBA7 output of the DC011 is also used to clock the NVR chip.
In most respects, the VT100 Technical Manual provides a phenomenal amount of detail.
However, documentation for some of the above ports is almost non-existent. It's only thanks to
[third parties](https://github.com/phooky/VT100-Hax/blob/master/Platform%20Notes.md) that we have, for example, the following
information about the Flags buffer (port 0x42):
Bit Active? Description
7 H KBD Transmit Buffer Empty
6 H LBA 7(?) (It's a pin on the backplane connector...) - used to clock NVR - line buffer address
5 H NVR DATA
4 L EVEN FIELD (comes out of the video timing generator)
3 H OPTION PRESENT (terminal output option???)
2 L GRAPHICS FLAG (is VT52 graphics card present) [I think he meant VT125 -JP]
1 L ADVANCED VIDEO (is AVO present)
0 H XMIT FLAG
VT100 Video Processor
---------------------