About my 8008/SCELBI OS/X Emulator Program

Things to Do or Fix

• Verify front panel functionality against actual hardware behavior
• Fix bug where serial port gets out of sync after interrupting or loading new program, after which the terminal becomes inoperable
• Add support for backspace on terminal
• Add support for additional emulated peripherials, such as paper tape, keyboard, oscilloscope display, cassette I/O
  

1. This version is an initial attempt at supporting front panel functionality.  Basics work about like a real SCELBI, but it hasn't been fine tuned and tested against the real thing, so more than likely, significant inconsistencies exist.  Use the SCELBI users manual from www.scelbi.com as a beginners guide to front panel operations.
   
2. There are 11 control switches on the SCELBI, 8 toggle switches for data bits B0-B7, and 3 momentary control buttons.
   
• The interrupt button stops the cpu and will cause the cpu to take the next instruction from switches  B0-B7 on the front panel (set switches B7 and B6 up and others down for NOP).  There is no reset input on the 8008 processor.
  
• The run button causes the cpu to start executing instructions until a halt instruction is executed or the user pushes the interrupt button.
• The step button causes the cpu to advance to the next T1 cycle (the current address is latched into address latches and CPU about to read or write from data bus).
• Set B0-B7 toggle switches up for a 1 and down for a 0.
  
3. Execution of  8008 instructions that access memory multiple times has been significantly revised in order to make the panel work.  Hopefully nothing was broken.
4. While the simulated 8008 is running, LED updates occur every 5 accesses.  The reduced LED update rate and the mechanism for scheduling simulated 8008 instructions isn't quite the same as the real thing,  but it gives you a fairly good feeling of what an actual SCELBI front panel looks like while running a program. 
   
5. Be aware that the LED updates put a fairly high load on your host CPU.  The 2GHz i7 CPU I developed this on shows about 35% CPU on activity monitor.
   
6. PC defaults to 00-100, so you can load SCELBAL into memory, using the menu and run without knowing how to work a SCELBI front panel, in order to run basic.
   

1. Mark Arnold (one of the original authors of SCELBAL) has fixed some mistakes in the version of SCEBAL I was originally using.  This mistakes were introduced during the reconstruction of the program.
2. I have made changes to AS8 to support the RST instruction and my interpretation of how OUT port numbering should be implemented.  If you are interested in this version, drop me an email.
3. The SCEBLI expects that all inputted characters have the high bit set - and starshooter and SCEBAL have been reassembled to match.  The power function such as 2^3 did not work in SCELBAL prior to this change.
4. The emulator itself now accurately counts cycles based on information gleaned from the 8008 data sheet.  Each cycle takes 2 clocks and instructions take from 3 to 11 cycles.  The emulator executes 250,000 cycles per second, same as an actual 8008.  
5. This version of the emulator uses port numbers to select serial or byte I/O, so the byte I/O menu has been removed.  This effectively emulates a SCELBI with both serial and parallel (BYTE I/O) ports connected.  See table below.

At this time, there are 2 working drop down menu items available with this application.

1. The Memory->Clear menu function will clear 8008 memory to zeros
2. The Memory->Load menu function can be used to load 8008 memory with an intel hex formatted file. I have a copy of scelbal (SCELBI Basic) in this format available to download. Other 8008 programs could be loaded, but the current assumption is that the PC is set to location 00-000 00-100 during reset.
3. The terminal section at the bottom of the window emulates either a UART or a bit banged serial interface compatible with drivers commonly found with Scelbal. Input is expected to be in upper case. If this window does not respond to input, either the 8008 is stopped, or not currently monitoring the serial port or you have found a bug in the app. Note that unlike a real serial port, input to the port from the keyboard is queued to a length of 4K characters to allow cutting and pasting input from other applications into the emulator. Overflow conditions are not currently handled gracefully.

The SCELBI/8008 emulation app can be downloaded from here.  Ty it for yourself, but keep in mind that the app is an experiment. It is likely to behave unpredictably, hang and/or crash from time to time.

The performance is throttled to approximately, the actual speed of a 500kHZ 8008. Load Basic and try executing a “FOR” loop of 0 to 100 to get an idea of the incredibly slow performance of an 8008 running Scelbal. The first version of the 8008 emulator that I had running in a Unix shell environment was approximately 500 times faster than an actual 8008, and even that felt slow!

Your host computer running the SCELBI application may get real busy, go to sleep or suspends for a while.  At this point, the  emulator will fall behind real time.  When this happens, the emulator will limit the catching up of the 8008 to actual elapsed real time to a 5 seconds burst of cycles.  In other words, if you put your computer to sleep for an hour, when it wakes up, the SCELBI application will give the emulated 8008 the maximum burst of 5 seconds worth of cycles and other cycles will be lost.

Have fun with it and be sure to let me know what you think.

Running SCELBAL (SCELBI Basic)

General information on SCELBAL and tiny SCELBAL can be found on this page.

To run Scelbal (basic), do the following.

1. Download and unzip the SCELBI application
   
2. Run the SCELBI app
3. Clear 8008 memory (optional)
4. Load 8008 memory with Scelbal.hex
5. Click on the “RUN” button – you should see a “READY” message
6. Type “SCR” – this clear’s Scelbal variables – if you don’t do this before entering your program, weird things will start happening
7. You can now enter a basic program or basic commands directly

Be aware that there are several common 2 character error messages in Scelbal including “BG” for two big and “SY” for syntax error.

For fun, try booting up Scebal and running this program:http://99-bottles-of-beer.net/language-scelbal-2472.html.  Once you have a the Scelbal ready prompt on your machine, copy and paste the source into the terminal section the SCELBI application.  Then type run to run it. 

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