[ned1] / software / 4func_calculator / calc.asm

# (c) 2018 Aaron Taylor <ataylor at subgeniuskitty dot com>
# See LICENSE.txt file for copyright and license details.

# Simple 4-function calculator in NED assembly.
# 20181128 - Aaron Taylor

calc
    JSR>calcinit

    calcloop
        # Refresh the display.
        JSR>printtopstackentry

        # Get a character/command.
        JSR>getchar

        # Check for 'zero stack entry' command.
        LDSP+0
        JSR>evalzerostackentry

        # Check for 'negate stack entry' command.
        LDSP+0
        JSR>evalnegatestackentry

        # Check for stack navigation commands.
        LDSP+0
        JSR>evalstacknavigation

        # Check for ASCII digit (append to stack entry) command.
        LDSP+0
        JSR>evalasciidigit

        # Check for ASCII 'addition' command.
        LDSP+0
        JSR>evalmathaddition

        # Check for ASCII 'subtraction' command.
        LDSP+0
        JSR>evalmathsubtraction

        # Check for ASCII 'multiplication' command.
        LDSP+0
        JSR>evalmathmultiplication

        # Check for ASCII 'division' command.
        LDSP+0
        JSR>evalmathdivision

        # Dump existing character and get the next character.
        TEST
        JMP>calcloop

# Should be unreachable.
HALT

##########################################################################################
calcinit
##########################################################################################
# Description:
#   Initializes stack for calc subroutine.
#   Zeros the Data Stack in RAM.
# Call Stack:
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Base Address (=0x40000000)
#   Data Stack Offset (=0)       <-- TOS
##########################################################################################
    # Setup program stack.
    WORD_1073741824
    SWAP
    IM_0
    SWAP

    # Prepare to generate Data Stack pointers.
    LDSP+2 # Data Stack Base Address
    LDSP+2 # Data Stack Offset

    # Zero the Data Stack.
    calcinitzeroloop

        # Increment Data Stack Offset, store 0 at location.
        JSR>incrementstackindex
        IM_0
        LDSP+2
        LDSP+2
        ADD
        STORE

        # Check for loop termination
        LDSP+0
        BRZ>calcinitzeroloopend
            JMP>calcinitzeroloop

    calcinitzeroloopend

    # Clean up stack and return.
    TEST
    TEST
    RTS

##########################################################################################
incrementstackindex
##########################################################################################
# Description:
#   Increment the Data Stack Offset, wrapping if approriate.
# Call Stack:
#   Data Stack Offset
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Offset
##########################################################################################
    # Increment the Data Stack Offset
    SWAP
    IM_4 # Four bytes per word
    ADD

    # See if the new Data Stack Offset should wrap. If so, set it to zero.
    LDSP+0
    IM_12       # Total of four stack entries (offsets: 0, 4, 8, 12).
    JSR>subtract
    TEST
    IM_12       # Address of PSW register.
    LOAD
    IM_2        # Negative bit in PSW
    AND
    BRZ>incrementstackindexreturn
        # Negative bit was set, so wrap to start of stack.
        TEST
        IM_0
    incrementstackindexreturn
    SWAP
    RTS

##########################################################################################
decrementstackindex
##########################################################################################
# Description:
#   Decrement the Data Stack Offset, wrapping if approriate.
# Call Stack:
#   Data Stack Offset
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Offset
##########################################################################################
    # Decrement the Data Stack Offset
    SWAP
    IM_4
    SWAP
    JSR>subtract

    # See if new Data Stack Offset should wrap, indicated by value < 0.
    # If so, set it to 12 (offsets: 0, 4, 8, 12).
    LDSP+0
    TEST
    IM_12   # Address of PSW register.
    LOAD
    IM_2    # Negative bit in PSW register.
    AND
    BRZ>decrementstackindexreturn
        # Negative bit was set, so wrap to end of stack.
        TEST
        IM_12
    decrementstackindexreturn
    SWAP
    RTS

##########################################################################################
printstackindex
##########################################################################################
# Description:
#   Prints the data stack index in user readable form.
# Call Stack:
#   Data Stack Offset
#   Return PC <-- TOS
# Return Stack:
#   <empty>
##########################################################################################
    # Put Return PC on bottom of stack.
    SWAP

    # Put post-number portion of the string on the stack (colon, space, NUL).
    IM_0    # ASCII NUL
    SWAP
    WORD_32 # ASCII ' '
    SWAP
    WORD_58 # ASCII ':'
    SWAP

    # Stack now looks like:
    #   Return PC
    #   ASCII NUL
    #   ASCII ' '
    #   ASCII ':'
    #   Data Stack Offset

    # Divide Data Stack Offset by 4 to generate index since there are 4 bytes per word.
    IM_4
    JSR>divide
    TEST # DROP Remainder

    # Convert index to ASCII and print the string.
    JSR>itoa
    JSR>printstring
    RTS

##########################################################################################
printbinaryinteger
##########################################################################################
# Description:
#   Prints a binary integer to ASCII terminal.
# Call Stack:
#   Integer
#   Return PC <-- TOS
# Return Stack:
#   <empty>
##########################################################################################
    # Put Return PC on bottom of stack.
    SWAP

    # Put ASCII NUL on stack as end of string.
    IM_0
    SWAP

    # Separate Integer into sign and magnitude.
    LDSP+0
    IM_4
    LOAD
    AND
    SWAP
    JSR>absolutevalue

    # Stack now looks like:
    #   Return PC
    #   ASCII NUL
    #   Sign flag (nonzero for negative result, 0 for positive result)
    #   ABS(Integer)

    # First check if the number is zero and print a single zero if true.
    LDSP+0
    BRZ>printbinaryintegerloopend

    # Repeatedly divide by ten and push each digit onto the stack in ASCII.
    printbinaryintegerloop

    # Verify Integer still has digits to print (i.e. Integer != 0)
    LDSP+0
    BRZ>printbinaryintegerloopend

        # Extract the least significant digit.
        IM_10
        JSR>divide

        # Convert to ASCII and add to string on stack.
        JSR>itoa
        LDSP+2 # Sign flag
        SWAP
        STSP+2
        SWAP

        # Loop
        JMP>printbinaryintegerloop

    printbinaryintegerloopend
    TEST # DROP ABS(Integer)

    # Push a leading ASCII zero onto stack.
    IM_0
    JSR>itoa
    SWAP

    # Stack now looks like:
    #   Return PC
    #   ASCII NUL
    #   ASCII digit
    #   ...
    #   ASCII digit
    #   Sign flag (nonzero for negative result, 0 for positive result)

    # Push sign onto stack as ASCII character.
    BRZ>printbinaryintegerpositive

        # Add ASCII '-' sign to stack
        WORD_44
        IM_1
        ADD
        JMP>printbinaryintegerend

    printbinaryintegerpositive

        # Add ASCII '+' sign to stack
        WORD_42
        IM_1
        ADD

    printbinaryintegerend

    # Print the string and return
    JSR>printstring
    RTS

##########################################################################################
printtopstackentry
##########################################################################################
# Description:
#   Prints the current TOS entry.
# Call Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   Return PC               <-- TOS
# Return Stack:
#   Data Stack Base Address
#   Data Stack Offset       <-- TOS
##########################################################################################
    # Prepare the screen and cursor.
    JSR>ansiescapeclearscreen
    JSR>ansiescapesetcursorcolumnone

    # Print the data stack index
    LDSP+1
    JSR>printstackindex

    # Print the TOS entry from the data stack in human readable form.
    LDSP+2
    LDSP+2
    ADD
    LOAD
    JSR>printbinaryinteger
    RTS

##########################################################################################
evalzerostackentry
##########################################################################################
# Description:
#   If character on TOS is ASCII "'", zero the current data stack entry.
# Call Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
#   ASCII character
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
##########################################################################################
    SWAP

    # Test for ASCII "'".
    WORD_38 # ASCII "'"
    IM_1
    ADD
    JSR>subtract
    BRZ>evalzerostackentrymatch

        # No match, return from subroutine
        RTS

    evalzerostackentrymatch

        # Matched, zero the stack entry and return.

        # Stack now looks like:
        #   Data Stack Base Address
        #   Data Stack Offset
        #   ASCII character
        #   Return PC <-- TOS

        IM_0
        LDSP+4 # Data Stack Base Address
        LDSP+4 # Data Stack Offset
        ADD
        STORE
        RTS

##########################################################################################
evalstacknavigation
##########################################################################################
# Description:
#   If character on TOS is ASCII '.' or ',', inc/dec Data Stack Offset to next entry.
# Call Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
#   ASCII character
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
##########################################################################################
    SWAP
    LDSP+0

    # Test for ASCII ','
    WORD_44 # ASCII ','
    JSR>subtract
    BRZ>evalstacknavigationprevmatch

        # No match.
        JMP>evalstacknavigationnexttest

    evalstacknavigationprevmatch

        # Matched, decrement Data Stack Offset.

        # Stack now looks like:
        #   Data Stack Base Address
        #   Data Stack Offset
        #   ASCII character
        #   Return PC <-- TOS
        #   ASCII character

        # Decrement the Data Stack Offset
        LDSP+3 # Data Stack Offset
        JSR>decrementstackindex
        STSP+3

        # Clean up stack and return.
        TEST
        RTS

    evalstacknavigationnexttest
    # Test for ASCII '.'
    WORD_46 # ASCII '.'
    JSR>subtract
    BRZ>evalstacknavigationnextmatch

        # No match.
        RTS

    evalstacknavigationnextmatch

        # Matched, increment Data Stack Offset

        # Increment and return.
        LDSP+2 # Data Stack Offset
        JSR>incrementstackindex
        STSP+2
        RTS

##########################################################################################
evalnegatestackentry
##########################################################################################
# Description:
#   If character on TOS is ASCII ';', negate the current data stack entry.
# Call Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
#   ASCII character
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
##########################################################################################
    SWAP

    # Test for ASCII ';'
    WORD_58 # ASCII ';'
    IM_1
    ADD
    JSR>subtract
    BRZ>evalnegatestackentrymatch

        # No match, return from subroutine
        RTS

    evalnegatestackentrymatch

        # Matched, negate the stack entry and return.

        # Stack now looks like:
        #   Data Stack Base Address
        #   Data Stack Offset
        #   ASCII character
        #   Return PC <-- TOS

        LDSP+3 # Data Stack Base Address
        LDSP+3 # Data Stack Offset
        ADD
        LDSP+0
        LOAD
        JSR>negate
        SWAP
        STORE
        RTS

##########################################################################################
evalmathaddition
##########################################################################################
# Description:
#   If character on TOS is ASCII '+', perform addition on TOS and NOS.
# Call Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
#   ASCII character
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
##########################################################################################
    SWAP

    # Test for ASCII '+' (43).
    IM_30
    IM_13
    ADD
    JSR>subtract
    BRZ>evalmathadditionmatch

        # No match, return from subroutine
        RTS

    evalmathadditionmatch

        # Matched. Perform addition.

        # Stack now looks like:
        #   Data Stack Base Address
        #   Data Stack Offset
        #   ASCII character
        #   Return PC <-- TOS

        # Fetch the first operand.
        LDSP+3  # Data Stack Base Address
        LDSP+3  # Data Stack Offset
        ADD
        LOAD

        # Decrement Data Stack Offset.
        LDSP+3  # Data Stack Offset
        JSR>decrementstackindex
        STSP+3

        # Fetch the second operand.
        LDSP+4  # Data Stack Base Address
        LDSP+4  # Data Stack Offset
        ADD
        LOAD

        # Perform the addition.
        ADD

        # Store the result.
        LDSP+4  # Data Stack Base Address
        LDSP+4  # Data Stack Offset
        ADD
        STORE

        # Return
        RTS

##########################################################################################
evalmathsubtraction
##########################################################################################
# Description:
#   If character on TOS is ASCII '-', perform subtraction NOS-TOS.
# Call Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
#   ASCII character
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
##########################################################################################
    SWAP

    # Test for ASCII '-' (45).
    IM_30
    IM_15
    ADD
    JSR>subtract
    BRZ>evalmathsubtractionmatch

        # No match, return from subroutine
        RTS

    evalmathsubtractionmatch

        # Matched. Perform subtraction.

        # Stack now looks like:
        #   Data Stack Base Address
        #   Data Stack Offset
        #   ASCII character
        #   Return PC <-- TOS

        # Fetch the first operand.
        LDSP+3  # Data Stack Base Address
        LDSP+3  # Data Stack Offset
        ADD
        LOAD

        # Decrement Data Stack Offset.
        LDSP+3  # Data Stack Offset
        JSR>decrementstackindex
        STSP+3

        # Fetch the second operand.
        LDSP+4  # Data Stack Base Address
        LDSP+4  # Data Stack Offset
        ADD
        LOAD

        # Perform the subtraction.
        JSR>subtract

        # Store the result.
        LDSP+4  # Data Stack Base Address
        LDSP+4  # Data Stack Offset
        ADD
        STORE

        # Return
        RTS

##########################################################################################
evalmathmultiplication
##########################################################################################
# Description:
#   If character on TOS is ASCII '*', perform multiplication on TOS and NOS.
# Call Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
#   ASCII character
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
##########################################################################################
    SWAP

    # Test for ASCII '*' (42).
    IM_30
    IM_12
    ADD
    JSR>subtract
    BRZ>evalmathmultiplicationmatch

        # No match, return from subroutine
        RTS

    evalmathmultiplicationmatch

        # Matched. Perform multiplication.

        # Stack now looks like:
        #   Data Stack Base Address
        #   Data Stack Offset
        #   ASCII character
        #   Return PC <-- TOS

        # Fetch the first operand.
        LDSP+3  # Data Stack Base Address
        LDSP+3  # Data Stack Offset
        ADD
        LOAD

        # Decrement Data Stack Offset.
        LDSP+3  # Data Stack Offset
        JSR>decrementstackindex
        STSP+3

        # Fetch the second operand.
        LDSP+4  # Data Stack Base Address
        LDSP+4  # Data Stack Offset
        ADD
        LOAD

        # Perform the multiplication.
        JSR>multiply

        # Store the result.
        LDSP+4  # Data Stack Base Address
        LDSP+4  # Data Stack Offset
        ADD
        STORE

        # Return
        RTS

##########################################################################################
evalmathdivision
##########################################################################################
# Description:
#   If character on TOS is ASCII '/', perform division NOS/TOS.
#   Returns quotient on TOS and remainder on NOS.
# Call Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
#   ASCII character
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
##########################################################################################
    SWAP

    # Test for ASCII '/' (47).
    IM_30
    IM_17
    ADD
    JSR>subtract
    BRZ>evalmathdivisionmatch

        # No match, return from subroutine
        RTS

    evalmathdivisionmatch

        # Matched. Perform division.

        # Stack now looks like:
        #   Data Stack Base Address
        #   Data Stack Offset
        #   ASCII character
        #   Return PC <-- TOS

        # Fetch the first operand.
        LDSP+3  # Data Stack Base Address
        LDSP+3  # Data Stack Offset
        ADD
        LOAD

        # Decrement Data Stack Offset.
        LDSP+3  # Data Stack Offset
        JSR>decrementstackindex
        STSP+3

        # Fetch the second operand.
        LDSP+4  # Data Stack Base Address
        LDSP+4  # Data Stack Offset
        ADD
        LOAD

        # Perform the division.
        SWAP
        JSR>divide

        # Store the remainder.
        LDSP+5  # Data Stack Base Address
        LDSP+5  # Data Stack Offset
        ADD
        STORE

        # Increment the Data Stack Offset
        LDSP+3  # Data Stack Offset
        JSR>incrementstackindex
        STSP+3

        # Store the quotient.
        LDSP+4  # Data Stack Base Address
        LDSP+4  # Data Stack Offset
        ADD
        STORE

        # Return
        RTS

##########################################################################################
evalasciidigit
##########################################################################################
# Description:
#   If character on TOS is ASCII digit, append it to current stack entry.
# Call Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
#   ASCII character
#   Return PC <-- TOS
# Return Stack:
#   Data Stack Base Address
#   Data Stack Offset
#   ASCII character
##########################################################################################
    SWAP

    # Test for ASCII digit.
    JSR>isasciidigit
    BRZ>evalasciidigitmatch

        # No match, return from subroutine
        RTS

    evalasciidigitmatch

        # Matched, append result to current stack entry and return.

        # Stack now looks like:
        #   Data Stack Base Address
        #   Data Stack Offset
        #   ASCII digit
        #   Return PC <-- TOS

        # Load the existing entry from top of data stack.
        LDSP+3 # Data Stack Base Address
        LDSP+3 # Data Stack Offset
        ADD
        LDSP+0
        LOAD

        # Multiply existing entry by ten and add new digit to least-significant location.
        IM_10
        JSR>multiply
        LDSP+3 # ASCII digit
        JSR>atoi
        ADD

        # Put new entry back on top of data stack and return.
        SWAP
        STORE
        RTS

##########################################################################################
isasciidigit
##########################################################################################
# Description:
#   Tests if character is ASCII digit (0..9). Returns 0 if true, 1 if false.
# Call Stack:
#   ASCII character
#   Return PC <-- TOS
# Return Stack:
#   <boolean, 0=true, 1=false>
##########################################################################################
    # Subtract ASCII '0' value, translating ASCII digit to integer.
    SWAP
    WORD_48         
    SWAP
    JSR>subtract
    # Copy and test negative result. This would indicate an ASCII character below '0'.
    LDSP+0
    TEST
    IM_12   # Address of PSW register
    LOAD
    IM_2
    AND
    BRZ>isasciidigitcontinued
        # The result was negative, so clean up stack and return false.
        IM_1
        STSP+0
        SWAP
        RTS
    isasciidigitcontinued
    # Subtract another 10 and check for positive result. This indicates ASCII char > '9'.
    IM_10
    SWAP
    JSR>subtract
    TEST
    IM_12
    LOAD
    IM_2
    AND
    BRZ>isasciidigitfalse
        # The result was true, so clean up stack and return true.
        IM_0
        SWAP
        RTS
    isasciidigitfalse
    # The result was false, so clean up stack and return false.
    IM_1
    SWAP
    RTS

##########################################################################################
generatesignflag
##########################################################################################
# Description:
#   Given a pair of twos-complement signed integers X and Y, generates a sign flag.
#   Flag is non-zero if the product of X and Y would be negative, otherwise flag is zero.
# Call Stack:
#   Y Operand
#   X Operand
#   Return PC <-- TOS
# Return Stack:
#   Sign Flag <-- TOS
##########################################################################################
    # Place Return PC at bottom of stack.
    LDSP+2
    LDSP+1
    STSP+3
    STSP+0
    
    # Stack now looks like:
    #   Return PC
    #   X Operand
    #   Y Operand <-- TOS

    IM_4
    LOAD
    AND
    SWAP
    IM_4
    LOAD
    AND
    XOR
    SWAP
    RTS
    
##########################################################################################
negate
##########################################################################################
# Description:
#   Returns the additive inverse of a twos-complement operand.
# Call Stack:
#   Operand
#   Return PC <-- TOS
# Return Stack:
#   Result <-- TOS
##########################################################################################
    SWAP
    NOT
    IM_1
    ADD
    SWAP
    RTS

##########################################################################################
absolutevalue
##########################################################################################
# Description:
#   Returns the absolute value of a twos-complement operand.
# Call Stack:
#   Operand
#   Return PC <-- TOS
# Return Stack:
#   Result <-- TOS
##########################################################################################
    SWAP
    LDSP+0
    IM_4
    LOAD
    AND
    BRZ>absolutevaluereturn
        JSR>negate

    absolutevaluereturn
        SWAP
        RTS

##########################################################################################
multiply
##########################################################################################
# Description:
#   Performs X*Y and returns result on TOS.
# Call Stack:
#   Y Operand
#   X Operand
#   Return PC <-- TOS
# Return Stack:
#   Result <-- TOS
##########################################################################################
    # Place Return PC at bottom of stack.
    LDSP+2
    LDSP+1
    STSP+3
    STSP+0
    
    # Stack now looks like:
    #   Return PC
    #   X Operand
    #   Y Operand <-- TOS

    # Generate a sign flag and store it behind the operands.
    LDSP+1
    LDSP+1
    JSR>generatesignflag
    LDSP+2
    SWAP
    STSP+2

    # Stack now looks like:
    #   Return PC
    #   Sign flag (nonzero for negative result, 0 for positive result)
    #   Y Operand
    #   X Operand <-- TOS

    # Convert both X and Y Operands to absolute value.
    JSR>absolutevalue
    SWAP
    JSR>absolutevalue

    # Stack now looks like:
    #   Return PC
    #   Sign flag (nonzero for negative result, 0 for positive result)
    #   ABS(X Operand)
    #   ABS(Y Operand) <-- TOS

    # Prepare the stack for multiplication algorithm
    IM_0                    # For magnitude of left/right shifts.
    LDSP+0                  # Sets up stack location to build/store result.

    # Check Nth bit of second operand.
    testbit
        LDSP+2              # Y Operand
        LDSP+2              # Shift magnitude
        SHIFT
        IM_1
        AND
        BRZ>skipadd
            # If indicated by a 1 bit, shift and add first operand to result.
            LDSP+3          # X Operand
            IM_4            # 0x80000000
            LOAD
            LDSP+3          # Shift magnitude
            OR
            SHIFT
            ADD
        skipadd

    # Increment shift magnitude counter.
    LDSP+1                  # Shift magnitude
    IM_1
    ADD
    STSP+1

    # Test for completion of multiplication algorithm (31 shifts).
    LDSP+1                  # Shift magnitude
    IM_30
    JSR>subtract
    BRZ>multiplycleanup
        JMP>testbit

    # Clean up the the stack after performing multiplication.
    multiplycleanup
        STSP+0
        STSP+0
        STSP+0

    # Stack now looks like:
    #   Return PC
    #   Sign flag (nonzero for negative result, 0 for positive result)
    #   ABS(X*Y) <-- TOS

    # Set the sign of the product.
    applysign
        SWAP
        BRZ>multiplyreturn
            JSR>negate

    # Clean up and return.
    multiplyreturn
        SWAP
        RTS

##########################################################################################
divide
##########################################################################################
# Description:
#   Division with remainder.
#   Halts on zero divisor.
# Call Stack:
#   Dividend
#   Divisor
#   Return PC <-- TOS
# Return Stack:
#   Quotient
#   Remainder <-- TOS
##########################################################################################
    # Move Return PC to bottom of stack.
    LDSP+2
    SWAP
    STSP+2
    SWAP

    # Stack now looks like:
    #   Return PC
    #   Dividend
    #   Divisor <-- TOS

    # Check for zero divisor
    LDSP+0
    BRZ>divideexception

    # Generate a sign flag and store it behind the operands.
    LDSP+1
    LDSP+1
    JSR>generatesignflag
    LDSP+2
    SWAP
    STSP+2

    # Stack now looks like:
    #   Return PC
    #   Sign flag (nonzero for negative result, 0 for positive result)
    #   Divisor
    #   Dividend <-- TOS

    # Convert both Dividend and Divisor to absolute value.
    JSR>absolutevalue
    SWAP
    JSR>absolutevalue

    # Stack now looks like:
    #   Return PC
    #   Sign flag (nonzero for negative result, 0 for positive result)
    #   ABS(Dividend)
    #   ABS(Divisor) <-- TOS

    # Prepare stack for division algorithm.
    IM_31           # Cycle Counter - Loop from n-1 -> 0 where n = 32 bits.
    IM_0            # Quotient
    IM_0            # Remainder

    # Binary long division
    divisionloop
        # Check Cycle Counter for end-of-loop condition (CC = -1).
        LDSP+2 # Cycle Counter
        IM_1
        ADD
        BRZ>divisionloopend

            # While Cycle Counter >= 0

            # Left-shift Remainder by 1 bit.
            IM_1
            IM_4 # Address of 0x80000000 register
            LOAD
            OR
            SHIFT
    
            # Set LSB of Remainder equal to bit (Cycle Counter) of Dividend.
            LDSP+4 # Dividend
            LDSP+3 # Cycle Counter
            SHIFT
            IM_1
            AND
            OR
    
            # Check if Remainder >= Divisor
            LDSP+3 # Divisor
            LDSP+1 # Remainder
            JSR>subtract
            TEST
            IM_12 # Address of PSW register
            LOAD
            IM_2 # Bit for Negative Flag in PSW
            AND
            IM_2
            XOR
            BRZ>divisionsubloopend

                # If Remainder >= Divisor

                # Set Remainder = Remainder - Divisor
                LDSP+3 # Divisor
                SWAP
                JSR>subtract

                # Set bit (Cycle Counter) of Quotient equal to 1.
                SWAP
                IM_1
                LDSP+3 # Cycle Counter
                IM_4
                LOAD
                OR
                SHIFT
                OR
                SWAP

            divisionsubloopend

            # Decrement Cycle Counter
            IM_1
            LDSP+3 # Cycle Counter
            JSR>subtract
            STSP+2

            # Loop over next division cycle
            JMP>divisionloop

        divisionloopend

        # Stack cleanup
        STSP+1
        STSP+1
        STSP+1

    # Stack now looks like:
    #   Return PC
    #   Sign flag (nonzero for negative result, 0 for positive result)
    #   Remainder
    #   Quotient <-- TOS

    # Set sign of results.
    LDSP+2 # Sign flag
    BRZ>divisioncleanup
        JSR>negate
        SWAP
        JSR>negate
        SWAP

    divisioncleanup

    # Clean up and return
    SWAP
    LDSP+3 # Return PC
    SWAP
    STSP+2
    SWAP
    STSP+2
    RTS

    # For now we can only HALT on errors and let the PC inform our debugging.
    divideexception
        HALT

##########################################################################################
subtract
##########################################################################################
# Description:
#   Performs X-Y and returns result on TOS.
# Call Stack:
#   Y Operand
#   X Operand
#   Return PC <-- TOS
# Return Stack:
#   Result <-- TOS
##########################################################################################
    # Place Return PC at bottom of stack.
    LDSP+2
    LDSP+1
    STSP+3
    STSP+0
    # Stack now looks like:
    #   Return PC
    #   X Operand
    #   Y Operand <-- TOS

    JSR>negate
    ADD
    SWAP
    RTS

##########################################################################################
atoi
##########################################################################################
# Description:
#   Converts an ASCII decimal into the corresponding integer value.
#   No bounds checks; assumed to already be performed in isasciidigit().
# Call Stack:
#   Operand
#   Return PC <-- TOS
# Return Stack:
#   Result <-- TOS
##########################################################################################
    SWAP
    WORD_48
    SWAP
    JSR>subtract
    SWAP
    RTS

##########################################################################################
itoa
##########################################################################################
# Description:
#   Converts an integer (0..9) into the corresponding ASCII character.
# Call Stack:
#   Operand
#   Return PC <-- TOS
# Return Stack:
#   Result <-- TOS
##########################################################################################
    # Copy operand to TOS
    LDSP+1
    # Verify that operand < 10
    IM_10
    LDSP+1
    JSR>subtract
    TEST # Set PSW according to difference.
    # Branch if non-negative:
    IM_12 # Address of PSW
    LOAD
    IM_2
    AND
    BRZ>itoahalt
    # Verify that operand > -1
    LDSP+0
    TEST # Set PSW according to operand.
    # Branch if negative:
    IM_12
    LOAD
    IM_2
    AND
    IM_2
    XOR
    BRZ>itoahalt # Branch if operand was negative.
    # Convert the integer to its ASCII representation and return to caller.
    WORD_48
    ADD
    STSP+1
    RTS
    # Halt on error
    itoahalt
        HALT

##########################################################################################
putchar
##########################################################################################
# Description:
#   Writes one character to the terminal.
# Call Stack:
#   Character to write
#   Return PC <-- TOS
# Return Stack:
#   <empty>
##########################################################################################
    WORD_134217728      # XBUF
    WORD_134217732      # XCSR
    LDSP+3
    SWAP
    putcharloop
        LDSP+0
        LOAD
        BRZ>putcharloop
    TEST # Drop XCSR from stack
    SWAP
    STORE
    # Wrote the character. Clean up stack and return.
    STSP+0
    RTS

##########################################################################################
getchar
##########################################################################################
# Description:
#   Reads one character from the terminal.
# Call Stack:
#   Return PC <-- TOS
# Return Stack:
#   Character <-- TOS
##########################################################################################
    WORD_134217736      # RBUF
    WORD_134217740      # RCSR
    getcharloop
        LDSP+0
        LOAD
        BRZ>getcharloop
    LDSP+1
    LOAD
    # Found a character. Clean up stack and return.
    STSP+0
    STSP+0
    SWAP
    RTS

##########################################################################################
printstring
##########################################################################################
# Description:
#   Prints a null-terminated string located on the stack.
# Call Stack:
#   ASCII NUL
#   ASCII character
#   ...
#   ASCII character
#   Return PC <-- TOS
##########################################################################################
    SWAP
    LDSP+0
    BRZ>printstringreturn
    JSR>putchar
    JMP>printstring

    printstringreturn
        TEST
        RTS

##########################################################################################
ansiescapeclearscreen
##########################################################################################
# Description:
#   Clears the entire screen.
# Call Stack:
#   Return PC <-- TOS
# Return Stack:
#   <empty>
##########################################################################################
    # ASCII NUL
    WORD_0
    # ASCII 'J'
    WORD_74
    # ASCII '2'
    WORD_50
    # ASCII '['
    WORD_90
    IM_1
    ADD
    # ASCII ESC
    WORD_26
    IM_1
    ADD
    # Print string and return
    JSR>printstring
    RTS

##########################################################################################
ansiescapesetcursorcolumnone
##########################################################################################
# Description:
#   Reset cursor to column 1 of screen.
# Call Stack:
#   Return PC <-- TOS
# Return Stack:
#   <empty>
##########################################################################################
    # ASCII NUL
    WORD_0
    # ASCII 'G'
    WORD_70
    IM_1
    ADD
    # ASCII '1'
    WORD_48
    IM_1
    ADD
    # ASCII '['
    WORD_90
    IM_1
    ADD
    # ASCII ESC
    WORD_26
    IM_1
    ADD
    # Print string and return
    JSR>printstring
    RTS