AWOL:
I think you missed (again) the part that says the byte code goes in secondary storage, NOT RAM.
I'm just trying to imagine how to connect all these peripherals to Uno board...
I'm just trying to imagine how to connect all these peripherals to Uno board.
How many lines do you think an SD card takes?
AWOL:
How many lines do you think an SD card takes?
It seems it takes 3 pins.
it's already done http://www.themaximitecomputer.com/
best regards
Tsvetan
go to this website
you can build a simple computer using arduino uno and atmega1284p
microcontroller.
there are firmware(codes) for simple computer
you can download it as a .zip file
this is really working-200% sure
try that
have a nice day..... 
There are different AVR Forths available. It's a far better language than Basic.
Once the base Forth is loaded you want a terminal for human IO. The code you then add gets stored to flash, it's there when you power up again. You can run it all you want.
After close to 4 years of Basic I found Forth and it changed everything including how I wrote Basic. It packs tight (code takes little space) and runs fast and has very few rules at all.
Not familiar with what an Atmel microcontroller looks like under the hood, but with Microchip mcu's you can store data in Flash RAM. You don't want to store variable data there, because it's only guaranteed for 100,000 write cycles, but you could store the BASIC source there because it's being written at a human pace not a computer pace. So it will take you a while to rewrite the program code 100,000 times. Then the RAM size is just RAM size, if it's not big enough for variable data for a program written in BASIC, it's probably not going to be big enough for the same program written in C.
Agree also with GoForSmoke. Used Forth back in the 80's and 90's and for an interactive language in tight memory space, you really can't beat it.
Anything wrong with a Raspberry Pi? 
Nothing wrong with Raspberry Pi AFAIK, but OP wasn't asking about that. If I was going to recommend something besides an Arduino, I'd probably have him look at the Chip. https://www.kickstarter.com/projects/1598272670/chip-the-worlds-first-9-computer
Well, when I consider to add a graphical display, keyboard, mouse(?) and disk drives/memory cards, I'd choose a system with support for all that built in. The Pi also has more RAM.
Or for my kids I'd choose a mobile device with touch screen included, and add a BASIC interpreter app, or a ZX-81 or C-64 emulator.
But I suppose it shall be a father-and-son project, where the father enjoys cabling and coding, and the son is the lame excuse to the unattended spouse
Guy signs up to necro a 4 yr old thread of another guy who never posted again.
Nobody notices. Zzzzz
I am building a computer from Arduino.
It has a Ps2 keyboard adapter for the keyboard
It has a TFT LCD as a screen
You can use the EEPROM library to store data on the arduino, even after the power is shut off.
Side note: When the computer has the keyboard plugged in, it goes into "computer" mode. But I also made it so that when the keyboard is unplugged it goes into "mobile" mode where the display rotates to a vertical position. In mobile mode, it can call and text using a GSM shield.
I thought all this info would spark some ideas for you!