I was looking at the excellent front panel Altair 8800 clone made a few years ago by dankar (see https://github.com/dankar/altair8800), and it occurred to me that except for the RAM, the entire clone could be built on a single ESP8266 ESP-12e 4mB flash device, with "raw" Telnet as a bonus So, based on that code, I did it. The case shown is a too large because I repurposed it from another project, but it could have been made as small as 1.5 x 1.5 x 3 inches.
Differences from dankar's build:
1. Uses SPIFFS
2. Uses a 512k, RAM; the benefit of this is the 16-bit address, saving one byte every memory read/write
3. No front panel, but A and B "head load" LED's and a firmware-controlled on/off light.
5. Built-in MAX3232 based RS-232
6. Built-in WiFi and Telnet (it really isn't "telnet", it's RAW)
7. Supervisory system :
a. Menu driven
b. Settings (for baud, for example) saved in flash
c. Switches between RS-232 and Telnet (serial port then monitors Telnet use)
d. Adds a selectable Telnet timeout,
e. Adds selectable CR delays for slow printers with long new line times (300 baud operation without handshaking)
f. Limited VT100 support.
g. A selection of boot/application disks and paper tape/ROM BASIC's
h. Ability to restore damaged "disks" and tapes from "golden" copies
I haven't removed the front panel support (and have a larger one of these built with the 74HC595's, etc), but the "run/stop" problem remains. the RUN/STOP switch could be interrupt driven, but I might not have the pins for it.
Speed is fairly good, quite usable, but I haven't benchmarked it.
I will probably add a few goodies like AP/client select with user-entered SSID's/passwords in the future.
The ESP-32 would have enough internal RAM to do this without the need for SPI RAM; it would also be faster.
Surely it would be fast enough and have I/O for an interrupt driven front panel. Hmmm...
This is a fairly decent framework for emulation and I've been thinking about emulating the 8008 and PDP/8 the same way, which should be possible to do with on-chip RAM alone. My code works, but is total structural garbage and I don't want anyone to actually see it - next to dankar's, it looks like crap.
I actually used an Altair 8800 when it was a current machine, and it blows my mind that this was done with two active packages, a few resistors, LED's, regulators and capacitors - about six 21'st century dollars of electronics (probably $1.50 1970's dollars).