Vintage Computer Hardware 101

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Vintage Computer Hardware 101
Featuring the MITS Altair 680b
Bill Degnan
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MITS Altair 680b

• Introduced - 1976
• 6800 microprocessor
• 500 Khz (.5 Mhz) clock speed
• 1,024 bytes of RAM memory (1K)
• 256 byte PROM monitor (ACIA)
• 11" x 11" x 5" (small)

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Altair 680b Motherboard
Copy
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Altair 680b Motherboard
Copy
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Binary and Hexidecimal Review

• Dec Binary
  Hexidecimal
• 0 0000 0
• 1 0001 1
• 2 0010 2
• 3 0011 3
• 4 0100 4
• 5 0101 5
• 6 0110 6
• 7 0111 7
• 8 1000 8
• 9 1001 9
• 10 1010 A
• 11 1011 B
• 12 1100 C
• 13 1101 D
• 14 1110 E
• 15 1111 F

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The Front Panel Switches

• Used to load information into the microcomputer
  memory
• Examine the contents of the microcomputer's
  memory
• Control microcomputer operations
• 1 address lamp 1 bit of memory address
• Switch up "1" "on" / Switch down "0"
  "off
• Altair 680b has 16 address bits and 8 data bits
• Control Switches
• Reset - (re) initialize the system
• Halt/Run - Halt - stops MPU activity, allows
  data addressing and input
• Run - Fetch the instruction in the program
  counter and activate/continue
• Dep - Used to enter data into a specific address

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Operating the Front Panel

• When the "AC" light is on - you have power
  (unless the AC LED is bad)
• The ADDRESS and DATA lights are in HEX
• Each address is made up of 4 HEX numbers (A0
  through A15)
• Each byte of data is made up of 2 HEX numbers
  (D0 through D7)

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Operating the Front Panel Example Address F0C2

• A15 on (up)
• A14 on (up) BINARY 1111
• A13 on (up)
• A12 on (up)
• A11 off (down)
• A10 off (down) BINARY 0000
• A9 off (down)
• A8 off (down)
• A7 on (up)
• A6 on (up) BINARY 1100
• A5 off (down)
• A4 off (down)
• A3 off (down)
• A2 off (down) BINARY 0010
• A1 on (up)
• A0 off (down)

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Changing Contents of a Data Location

• 1. Halt
• 2. Position the address switches to the
  appropriate HEX address
• 3. Position the data switches to the desired
  HEX data value
• 4. Flick the DEP (deposit) switch to store.
• 5. Reset (re-starts program counter)
• 6. Run

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ROM section of mother board, ACIA
Copy
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How to Interpret Machine Language from Program
Listing

• Column 1 program line number
• Column 2 memory address in HEX
• Column 3 data value in memory address from
  column 2
• Column 4 operands. Data values in Next memory
  addresses, following column 2 "starting" address
• Column 5 Assembly Language and comments (not
  needed for front panel operations)
• Example Line from program listing 00044 FF08 F4
  F001
• HEX address FF08 data byte should be set to F4
• HEX address FF09 data byte should be set to F0
• HEX address FF0A data byte should be set to 01

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How to Test Machine Language from Program Listing
Using Toggle Switches

• 1. Halt
• 2. Position the address switches to desired HEX
  memory location matching the program code listing
• 3. The data lights should display what is
  currently stored with the address

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ACIA Asynchronous Communications Interface
Adapter

• "..data formatting and control to interface
  serial asynchronous data communications
  information to bus organized systems.."
• such as the Altair 680b 6800 processor.
• - Examine or change the contents of a memory
  location
• - Load paper tape into memory
• - Start a program at a selected address
• - Debugging
• ...But most important - No more toggle switching!
  Saves Time.

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Using ACIA Monitor from a Terminal

• 1. Set the address switches to FF02
• 2. Turn on the Altair 680b
• 3. Turn on the terminal (assuming it's cabled
  to your computer correctly)
• 4. Switch the RUN/HALT to "Halt"
• 5. Actuate the RESET switch
• 6. The ACIA PROM will respond by sending a
  CR/LF to the terminal and the
  terminal will display a dot prompt (.)

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ACIA Monitor Commands - M

• M - Memory Examine and Deposit
• 1. At the . prompt, type M (but do not press
  Enter)
• 2. The monitor will respond by printing a space
• 3. Enter the HEX memory location
• 4. The monitor will respond by first printing
  the data value in HEX stored in the memory
  location, followed by a space.
• 5. To change the contents of the byte, enter the
  new 2-digit HEX value (do not press Enter).
• 6. If you do not wish to change the value of the
  byte, press Enter without entering a new value
• THUS - This is the same thing as using the DEP
  toggle to change the value of data for a given
  HEX address.

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ACIA Monitor Commands - N

• N Memory Deposit and Examine Next
• 1. At the . prompt, type N (but to not press
  Enter)
• 2. The monitor will print the next memory
  address in sequence.
• 3. Just like the "M" command, you can opt to
  change the contents of the specified byte
  or leave as is.

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ACIA Monitor Commands - J

• J Jump to Specified Address
• 1. At the . prompt, type J (but to not press
  Enter)
• 2. The monitor will respond by printing a
  space
• 3. Enter the 4-digit HEX memory location (but
  don't hit enter)
• 4. The processor will jump to the specified
  memory location and start executing the
  program from that location.

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ACIA Monitor Commands - L

• L Load Paper Tape
• 1. At the . prompt, type L (but to not press
  Enter)
• 2. Place the paper tape in the reader, and
  start the reader.
• S0 comments
• S1 code
• S9 done

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ACIA Monitor Commands - P

• P Proceed to Program Breakpoint
• 1. At the . prompt, type P (but to not press
  Enter)
• 2. The program will continue.
• NOTE This is a debugging tool. The programmer
  sets breakpoints to halt a program and return
  control to the monitor prompt so that the memory
  can be evaluated.