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Utilities for Mano's Basic Computer

Links on this page:

Writing a String
Stacks

Push
Pop

Testing

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.

Writing a String

Code	Pseudo-code	Nearer assembly language	Comments
// date: Aug 8, 2012 // author: Nicholas Duchon // purpose: write an ascii string // in Mano basic computer // use: put the address of the // start of the string into AC, // -- String must end with a dot! // example of calling routine ORG 10 LDA STRLOC BSA WRTSTR HLT STRLOC HEX STRING // string addr STRING STR "ONE." // subroutine to write a string ORG 100 WRTSTR HEX 0 STA WRTSTRA // save str WRTSTR0 LDA WRTSTRA I // get word CIR // get high byte CIR CIR CIR CIR CIR CIR CIR AND BYTEMASK // see 1 byte ADD WRTSTRDOTN // dot? SZA BUN WRTSTR1 // not dot BUN WRTSTR I // dot > ret WRTSTR1 ADD WRTSTRDOT // restore OUT // write LDA WRTSTRA I // for low AND BYTEMASK // see 1 byte ADD WRTSTRDOTN // dot? SZA BUN WRTSTR2 // not dot BUN WRTSTR I dot > ret WRTSTR2 ADD WRTSTRDOT // restore OUT // write ISZ WRTSTRA // next word BUN WRTSTR0 // loop WRTSTRA HEX 0 WRTSTRDOT CHR '.' WRTSTRDOTN HEX FFD2 BYTEMASK HEX FF	`WRTSTR (strlocation) while true get char if char = dot return else write char end while end subroutine`	`WRTSTR <return address here> AC -> WRTSTRA <string pointer> // effectively i=0 loop: load AC with str[i] CIR x 8, move high byte to low mask ff, remove high byte add ascii for -dot if 0, return add ascii dot,restoring char write char load AC with str[i] mask ff // now low byte add ascii -dot if 0 return add ascii dot, restore char write char i < i+1 // next 2 chars go to loop`	A string is defined using the STR assembler directive, and for this subroutine to work correctly, the string should end with a dot (period). Since the computer uses 16-bit words, a string is packed 2 ASCII characters per word, so the subroutine needs to first print the upper byte, then the lower byte. Also the subroutine needs to check each byte to see if it is the dot, which will indicate the end of the string, and will not be printed. In the calling routine, the address of the start of the string must be placed in AC as the input parameter to the subroutine. Again because of the limitations of the machine language of Mano's computer, another labelled word needs to be declared which will hold the address of the start of the string, such as `STRLOC` in the example program.

Stacks

Code Pseudo-code Comments

// Code for stacks: // STKPUSH - push AC onto stack // STKPOP - pop top of stack to AC // WRTSTR - write string utility // code at 150+ // stack at 200 (STKTOS) // max elements 100 (STKMAX) ORG 150 STKPUSH HEX 0 STA STKTMP LDA STKCNT INC STA STKCNT ADD STKMAX SZA BUN STKP2 LDA STK2MSGL BSA WRTSTR HLT // overflow error STKP2 LDA STKTMP STA STKTOS I ISZ STKTOS BUN STKPUSH I // return STKPOP HEX 0 LDA STKCNT ADD NEGONE SNA BUN STKP3 LDA STK3MSGL BSA WRTSTR HLT STKP3 STA STKCNT LDA STKTOS ADD NEGONE STA STKTOS LDA STKTOS I BUN STKPOP I NEGONE DEC -1 STKTMP HEX 0 STKCNT HEX 0 STKMAX DEC -100 STKTOS HEX 200 // stack 200 start STK2MSGL HEX STK2MSG STK2MSG STR "Push stack overflow." STK3MSGL HEX STK3MSG STK3MSG STR "Pop stack underflow." // subroutine to write a string WRTSTR HEX 0 STA WRTSTRA // save str WRTSTR0 LDA WRTSTRA I // get word CIR // get high byte CIR CIR CIR CIR CIR CIR CIR AND BYTEMASK // see 1 byte ADD WRTSTRDOTN // dot? SZA BUN WRTSTR1 // not dot BUN WRTSTR I // dot > ret WRTSTR1 ADD WRTSTRDOT // restore OUT // write LDA WRTSTRA I // for low AND BYTEMASK // see 1 byte ADD WRTSTRDOTN // dot? SZA BUN WRTSTR2 // not dot BUN WRTSTR I // dot > ret WRTSTR2 ADD WRTSTRDOT // restore OUT // write ISZ WRTSTRA // next word BUN WRTSTR0 // loop WRTSTRA HEX 0 WRTSTRDOT CHR '.' WRTSTRDOTN HEX FFD2
BYTEMASK HEX FF

push (AC) counter ++ if (counter > max) error message halt end if TOS [counter] <-- AC TOS ++ return end push pop () counter -- if (counter < 0) error message halt end if AC <-- TOS [counter] return end pop

There are 3 subroutines in this code:

STKPUSH
STKPOP
WRTSTR

The last one is included to make it easier to copy/paste this code into my simulator. It is used by the push and pop routines to print error messages: stack overflow in push, and stack underflow in pop. Both of these errors cause the simulator to halt.

Push takes the value in AC and pushes it onto the stack, incrementing the number of elements in the stack (STKCNT) and incrementing the top of stack (TOS) pointer (STKTOS). TOS points to the next free location on the stack.

Pop takes the element at the top of the stack (TOS-1), returns it in the AC register, decrements the number of elements in the stack, and decrements the TOS.

ISZ can be used effectively for the increment operations, but since Mano's computer has not analogous instruction for decrement, subtracting one can only be done using the AC register, meaning that the contents of the AC are going to be lost, which in turn means that any code calling this routine will have to save the value in the AC register someplace safe before calling this routine the pop routine.

Testing

Code Comments

// testing stacks: push, pop // also testing WRTSTR ORG 10 CLA INC INC BSA STKPUSH // 1 INC BSA STKPUSH // 2 INC BSA STKPUSH // 3 INC BSA STKPUSH // 4 INC BSA STKPUSH // 5 INC BSA STKPUSH // 6 INC BSA STKPUSH // 7 INC BSA STKPUSH // 8 BSA STKPOP // 8 OUT BSA STKPOP // 7 OUT BSA STKPOP // 6 OUT BSA STKPOP // 5 OUT BSA STKPOP // 4 OUT BSA STKPOP // 3 OUT BSA STKPOP // 2 OUT BSA STKPOP // 1 OUT BSA STKPOP // 0 OUT BSA STKPOP // -1 OUT HLT This code will push 8 numbers onto the stack, then try to pop 10. The pop subroutine should complain when trying to pop the 9th element and halt the simulation. After each element is popped, the code will write the value of the element to the output, as a hex number, so to see the results, the output panel should be set to hex rather than ASCII.

The first number pushed is 2, then 3, 4, 5, 6, 7, 8 and 9, just to make the numbers a little interesting in the test.

end.

By Dr. Nicholas Duchon
Last update: Aug 11, 2012