Mano Examples

Examples of Programming
Mano's Basic Computer

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Dr. M. Moris Mano describes a basic computer in his textbook,
Computer System Architecture, 3rd edition, 1993.

This computer is a toy computer, but serves to present many of the basic concepts
behind all digital computers and
gives us a good platform to study some key features of assembly language programming.

The sample programs I have included in this tutorial
can be assembled and executed using My Computer Simulator.
Other Links:

GCD as a main program

Code
 Pseudo-code
 Nearer assembly language
 Comments
// Greatest common divisor
// code, following Stallings
// 9th, pg 706, without
// testing for 0 on A or B
// Some interesting test cases:
// decimal inputs, hex result
//  2100,  882 --> 2a
//  1972, 1218 --> 3a
//  3264, 5280 --> 60
//  3248, 5264 --> 70

     ORG 20
LOOP LDA AVAL
     CMA
     INC // GET -A
     ADD BVAL // B-A
     SZA
     BUN CONT
     BUN DONE
CONT SNA
     BUN BIGA
     LDA BVAL // DO A - B
     CMA
     INC
     ADD AVAL
     STA AVAL // A > B
     BUN LOOP
BIGA STA BVAL // B > A
     BUN LOOP
DONE LDA BVAL
     STA RET
     HLT

     ORG 40
AVAL DEC 3248
BVAL DEC 5264
RET  DEC 0
 while a != b
  if a < b
    b = b - a
  else
    a = a - b
  end else
end while
return b
 LOOP: compute b - a
      if result is 0, go to DONE
      if result < 0, go to BIGA
      store result in b
      go to LOOP
BIGA: compute a - b
      store result in a
      go to LOOP
DONE: put b into result
      halt

// code for A - B:
      LDA BVAL  // AC <- B
      CMA       // AC <- B'
      INC       // AC <-- (B'+1)
              // so, AC <-- (-B)
      ADD AVAL  // AC <- (-B + A)
 Because Mano's Basic Computer only has very limited arithmetic and branching capabilities, the code for calculating a subtraction operation and the code for branching get rather convoluted. The computer uses 2's complement to represent negative numbers, so a subtraction operation is implemented as follows:
    a - b = -b + a = (b' + 1) + a
Where b' is the bitwise complement of b.

Generally, assembly languages implement "goto" statements rather then structured programming concepts such as "if/then/else" and "while" loops. However, it is a good idea to simulate the logic of those structures in the assembly language as nearly as possible, which we have tried to do in the "Nearer" code - the third column.

When the "nearer" code refers to a result, the assembly language program is actually moving or testing the value in the AC register.

An additional limitation of this machine is that it only has "test and skip next instruction if true" branching, which complicates the overall structure of the code. Most real computers allow branching instructions to go nearly anywhere, giving much more compact code.

Input and Output

The first observation is that this program should continue running until the input character is a flag value, in this a period, '.'. The program will need to do a comparison, but the only way this can be done in Mano's computer is by adding two values and checking the result, which means that we need to add the character just read in to the "negative" of the target character. Thus, the program begins by computing that negative.

The "look for char" operation is implemented using the SKI test. My simulator will make this flag true and put data into the input register, INPR, if there are one or more characters in the Input panel.

This code also assumes that the output register, OUTR, works pretty much instantly. If there was any question, the code should be checking using SKO before it uses the OUT command.

The program uses a pointer, PTR, to point to the next free entry in the array of characters, uses indirect addressing to put the data into the memory location referenced by PTR, and uses the ISZ instruction to update the value in PTR. The result of this ISZ should always be false as long as the array of characters does not overflow the memory. Thus, there are limits to this program. Other than this overflow problem, using ISZ is a rather elegant solution to walking through the memory one word at a time.

This program loads chars from input stream, copies them to output stream, and to memory locations starting at 50, until the target char, ECH, encountered. It will infinite loop if the input stream does not end with a . - period.

Code
 Pseudo-code
 Nearer assembly language
    ORG 10
    LDA ECH   // calculates 2's comp
    CMA       // 1's comp
    INC       // 2's comp
    STA ECH   // stores 2's of target
FST SKI       // look for input
    BUN FST   // no input
    CLA       // clear AC
    INP       // input to AC
    OUT       // ouput char
    STA PTR I // store char
    ISZ PTR   // inc location
    ADD ECH   // is it target?
    SZA       // quit if target
    BUN FST   // next char
END HLT       // quit
ECH HEX 2E // end character "."
PTR HEX 50 // store chars here
    END   
do
  get char
  write char
  store char in array
while char != '.'

 LOOP:
   look for char
   get char
   write char
   store char in array
   if char != '.' go to LOOP
   halt

Subroutine A - Sum elements, global variables

For convenience, we will call the caller code "main". In all these cases, the subroutine will take 5 values and add them up, returning the result to main. 

Code
 Pseudo-code
 Nearer assembly language
     ORG 10
     BSA SUBA // call subroutine
     LDA SUM
     OUT      // output result
     HLT

SUBA HEX 0      // return address
     LDA NUMB
     CMA
     INC
     STA CNT    // put -NUMB to CNT
     LDA DATP
     STA PTR
     CLA        // so we can do it again
LOOP ADD PTR I  // add next value to AC
     ISZ PTR    // point at next element
     ISZ CNT    // increment counter
     BUN LOOP   // loop if cnt != 0
     STA SUM    // done, sum <- AC
     BUN SUBA I // return, indirect

     ORG 30     // temp vars here
NUMB DEC 5      // number to add up
DATP HEX DATA   // start of array
SUM  HEX 0      // result
PTR  HEX 0      // current location
CNT  HEX 0      // countup counter

     ORG 40 // data here
DATA DEC 1       //  [0], N =  1
     DEC 2       //  [1], N =  2
     DEC 4       //  [2], N =  3
     DEC 8       //  [3], N =  4
     DEC 16      //  [4], N =  5
     DEC 32      //  [5], N =  6
     DEC 64      //  [6], N =  7
     DEC 128     //  [7], N =  8
     DEC 256     //  [8], N =  9
     DEC 512     //  [9], N = 10
     DEC 1024    // [10], N = 11
     END   
START:
  call SUBA
  write sum
  halt

SUBA:
  sum = 0
  for i=0; i<numb; i++
    sum += DATA[i]
  end for
  return

DATA:
  <data here>
  • for translates this way:
ptr = 0
for i= -numb; i!=0; i++
  sum += DATA[ptr]
  ptr ++
end for


 START:
  call SUBA
  write SUM
  halt

SUBA:
  return address
  CNT <- -NUMB
  PTR <- DATA
  AC <- 0
LOOP:
  AC += [PTR]
  PTR ++
  CNT ++
  if CNT != 0 goto LOOP
  store AC to SUM
  return indirect

Variables (local/global)
  NUMB  // input to sub
  DATP  // input
  SUM   // output
  PTR   // local to sub
  CNT   // local

Data (input to sub)

[PTR] means the data at
memory location PTR

Subroutine B - Sum elements, parameters in caller

The only difference between this and Subroutine A is that the input and output parameters are located after the BSA call in the main program, and the subroutine accesses those parameters using indirect addressing, and uses the ISZ command to walk through the parameter list one at a time.

Eventually, the final ISZ is used to point to the next line of code in the main program, the line to which the subroutine should return.

Overall, this is one way to avoid the use of global variables. In this case, the subroutine uses the return address to get at the parameters, and the variables PTR and CNT should be considered local variables. The location SUBA, the start of the subroutine, must, of course, be known to main.

Code
     ORG 10
     BSA SUBA   // call subroutine
     DEC 5      // 1st param, number to add up
     HEX DATA   // 2nd param, start of array
SUM  HEX 0      // 3rd param, result
     LDA SUM
     OUT        // output result
     HLT

// subroutine's local variables:
PTR  HEX 0      // current location
CNT  HEX 0      // countup counter
SUBA HEX 0      // return address
// start of subroutine body
     LDA SUBA I // first parameter, number to add
     CMA
     INC
     STA CNT    // put -NUMB to CNT
     ISZ SUBA   // point at second parameter
     LDA SUBA I // get ptr to data
     STA PTR
     CLA        // so we can do it again
LOOP ADD PTR I  // add next value to AC
     ISZ PTR    // point at next element
     ISZ CNT    // increment counter
     BUN LOOP   // loop if cnt != 0
     ISZ SUBA   // point to 3rd parameter
     STA SUBA I // done, 3rd param <- AC
     ISZ SUBA   // point to return location
     BUN SUBA I // return, indirect

     ORG 40 // data here
DATA DEC 1       //  [0], N =  1
     DEC 2       //  [1], N =  2
     DEC 4       //  [2], N =  3
     DEC 8       //  [3], N =  4
     DEC 16      //  [4], N =  5
     DEC 32      //  [5], N =  6
     DEC 64      //  [6], N =  7
     DEC 128     //  [7], N =  8
     DEC 256     //  [8], N =  9
     DEC 512     //  [9], N = 10
     DEC 1024    // [10], N = 11
     END    

Subroutine C - Sum, parameters in subroutine

In this version, main will load the parameters into the local space of the subroutine, getting the values from main's local variable space. Then the value returned will be available to main in the AC register, so this subroutine will be acting more like what we traditionally call a function.

The code uses the variable SUBP to hold the address of the subroutine so that the parameters can be put after that location using indirect addressing, walking through the parameter list using the ISZ command on the PARP variable. We do not want to use PARP as the only variable pointing to SUBA because we might wish to call SUBA more than once, and because of the ISZ operations, PARP will not always point to the SUBA starting address.

The parameters are all accessed relative to the location of the start of the subroutine, SUBA.

Code
     ORG 10
     LDA SUBP   // get address of sub start
     STA PARP   // place to put parameters
     ISZ PARP   // skip return address location
     ISZ PARP   // skip first instruction, BUN CODE
     LDA NUMB   // 1st parameter, number
     STA PARP I // 1st parameter in sub
     ISZ PARP   // go to second parameter, data pointer
     LDA DATP
     STA PARP I // store second parameter
     BSA SUBA   // call subroutine
     STA SUM
     OUT        // output result
     HLT
// data local to main:
SUBP HEX SUBA   // address of subroutine
PARP HEX 0
NUMB DEC 5      // 1st param, number to add up
DATP HEX DATA   // 2nd param, start of array
SUM  HEX 0      // 3rd param, result

// subroutine's local variables:
SUBA HEX 0      // return address
     BUN CODE
LOCN HEX 0      // local copy of n, 1st param
LOCD HEX 0      // local copy of datap, 2nd param
PTR  HEX 0      // current location
CNT  HEX 0      // countup counter
// start of subroutine body
CODE LDA LOCN  // first parameter, number to add
     CMA
     INC
     STA CNT    // put -NUMB to CNT
     LDA LOCD   // get ptr to data, 2nd param
     STA PTR
     CLA        // so we can do it again
LOOP ADD PTR I  // add next value to AC
     ISZ PTR    // point at next element
     ISZ CNT    // increment counter
     BUN LOOP   // loop if cnt != 0
     BUN SUBA I // return, indirect

     ORG 40 // data here
DATA DEC 1       //  [0], N =  1
     DEC 2       //  [1], N =  2
     DEC 4       //  [2], N =  3
     DEC 8       //  [3], N =  4
     DEC 16      //  [4], N =  5
     DEC 32      //  [5], N =  6
     DEC 64      //  [6], N =  7
     DEC 128     //  [7], N =  8
     DEC 256     //  [8], N =  9
     DEC 512     //  [9], N = 10
     DEC 1024    // [10], N = 11
     END   

Subroutine D - Sum, parameters on stack - Coming

Code
 Pseudo-code
 Nearer assembly language
// Recursive function using stack
// sum 1 through n
// f(n) = n + f(n-1) for n > 0
//      = 0          for n = 0
//      = ?          for n < 0

// main program
            ORG 10
            LDA TARGET  // AC <-- n
            BSA RSUM    // AC <-- f(n)
            OUT         // write n, in hex
            HLT
TARGET      DEC 20

// subroutine

 main
  n = 20
  write (rsum(20))
end main


 main
  AC <-- n
  call RSUM
  write AC (rsum(n))
end main
n given value 20

Subroutine E - Sum 1 through n recursively

This is an illustration of using a stack to implement recursive function calls.

The program must assume that the values in the local variables and the AC will be corrupted by the function call, so any information this instantiation of the subroutine needs should be saved on the stack before the call, and restored from the stack afterwards. In this case, that information is the return address (which will always need to be saved) and the value of the parameter n. Thus, before the recursive call, these two pieces of information must be pushed onto the stack, and when the subroutine returns, two calls to pop the information from the stack are required.

The stacking subroutines (STKPUSH and STKPOP) can be found on the Utilities page of my notes.

A program this complex should have a number of test cases, which I have provided below, and since the output of the simulator is in hex, I have provided both decimal and hex results.

 n  f(n) dec   f(n) hex     n  f(n) dec   f(n) hex
 1      1         1        11     66        42
 2      3         3        12     78        4e
 3      6         6        13     91        5b
 4     10         a        14    105        69
 5     15         f        15    120        79
 6     21        15        16    136        88
 7     28        1c        17    153        99
 8     36        24        18    171        ab
 9     45        2d        19    190        be
10     55        37        20    210        d2

A question for the interested reader: since this computer uses 16-bit 2's complement, it can only hold positive numbers up to 215-1, so what is the biggest value of n for which this program will work correctly, ie, not result in an overflow?

Code
 Pseudo-code
 Nearer assembly language
// Recursive function using stack
// sum 1 through n
// f(n) = n + f(n-1) for n > 0
//      = 0          for n = 0
//      = ?          for n < 0

// main program
            ORG 10
            LDA TARGET  // AC <-- n
            BSA RSUM    // AC <-- f(n)
            OUT         // write n, in hex
            HLT
TARGET      DEC 20

// subroutine
RSUM        HEX 0        // return address
            SZA          // n = 0?
            BUN RSUMA    // n <> 0, so continue
            BUN RSUM I   // n = 0 so return 0 in AC
RSUMT       HEX 0        // local, transient, storage
RSUMA       STA RSUMT    // temp <-- n
            BSA STKPUSH  // stack <-- n
            LDA RSUM     // AC <-- return address
            BSA STKPUSH  // stack <-- return address
            LDA RSUMT    // AC <-- n
            ADD NEGONE   // AC <-- (n-1)
            BSA RSUM     // AC <-- f(n-1)
            STA RSUMT    // temp <-- f(n-1)
            BSA STKPOP   // AC <-- return address
            STA RSUM     // ret location <-- return address
            BSA STKPOP   // AC <-- n
            ADD RSUMT    // AC <-- n + f(n-1)
            BUN RSUM I   // return
 main
  n = 20
  write (rsum(20))
end main

function rsum (int n)
  if (n=0) return 0
  return n + rsum (n-1)
end function

 main
  AC <-- n
  call RSUM
  write AC (rsum(n))
end main
n given value 20

rsum:
   return address
   AC = 0?
     no - continue
     yes - return 0
temp:
   word
continue:
   temp <-- AC (n)
   stack AC (n)
   stack return address
   AC <-- temp
   AC <-- n-1
   call rsum // get f(n-1)
   temp <-- AC (f(n-1))
   pop to return address
   pop n to AC
   add f(n-1) to n (in AC)
   return
  
  


Subroutine F - Coming

Code
 Pseudo-code
 Nearer assembly language
    ORG 10
    LDA ECH   // calculates 2's comp
    CMA       // 1's comp
    INC       // 2's comp
    STA ECH   // stores 2's of target
FST SKI       // look for input
    BUN FST   // no input
    CLA       // clear AC
    INP       // input to AC
    OUT       // ouput char
    STA PTR I // store char
    ISZ PTR   // inc location
    ADD ECH   // is it target?
    SZA       // quit if target
    BUN FST   // next char
END HLT       // quit
ECH HEX 2E // end character "."
PTR HEX 50 // store chars here
    END   
do
  get char
  write char
  store char in array
while char != '.'

 LOOP:
   look for char
   get char
   write char
   store char in array
   if char != '.' go to LOOP
   halt
end.
By Dr. Nicholas Duchon
Last update: Aug 18, 2012