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Overview

The GeneradorASM (Assembly Generator) class generates x86 assembly code compatible with EMU8086 from the Abstract Syntax Tree. This is the sixth and final phase of compilation, producing executable machine code.

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Class Definition

class GeneradorASM:
    def __init__(self)

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Constructor Parameters

No parameters required. The generator initializes with empty code and variable lists.

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Attributes

  • codigo (List[str]): List of generated assembly instructions
  • variables (set): Set of variable names used in the program

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Public Methods

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generar()

Generates complete x86 assembly code from the AST. 
def generar(self, programa: Programa) -> str
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programa
Programa
required
The AST produced by the Parser
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return
str
Complete assembly source code as a string, ready to be assembled
Example: 
scanner = Scanner("let x = 5 + 3; print x;")
tokens = scanner.escanear_tokens()

parser = Parser(tokens)
programa = parser.parsear()

generador = GeneradorASM()
codigo_asm = generador.generar(programa)

print(codigo_asm)
# Produces complete .asm file content

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Generated Assembly Structure

The generated assembly follows the standard x86 small model structure: 
.model small
.stack 100h

.data
[variable declarations]
msg db 13,10,'$'

.code
main proc
mov ax, @data
mov ds, ax

[program instructions]

mov ah,4ch
int 21h
main endp

[print_num routine]

end main

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Assembly Sections

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Model and Stack

.model small    ; Small memory model
.stack 100h     ; 256 bytes stack

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Data Section

All variables are declared as 16-bit words: 
.data
x dw 0          ; Variable x
y dw 0          ; Variable y
result dw 0     ; Variable result
msg db 13,10,'$' ; Newline string

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Code Section

The main program logic: 
.code
main proc
mov ax, @data   ; Initialize data segment
mov ds, ax

; Program instructions here

mov ah,4ch      ; Exit program
int 21h
main endp

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Code Generation Examples

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Simple Assignment

# Source: let x = 10;
# Assembly:
mov ax, 10
mov x, ax

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Addition

# Source: let sum = 5 + 3;
# Assembly:
mov ax, 5
push ax
mov ax, 3
mov bx, ax
pop ax
add ax, bx
mov sum, ax

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Subtraction

# Source: let diff = 10 - 3;
# Assembly:
mov ax, 10
push ax
mov ax, 3
mov bx, ax
pop ax
sub ax, bx
mov diff, ax

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Multiplication

# Source: let prod = 6 * 7;
# Assembly:
mov ax, 6
push ax
mov ax, 7
mov bx, ax
pop ax
imul bx
mov prod, ax

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Division

# Source: let quot = 10 / 2;
# Assembly:
mov ax, 10
push ax
mov ax, 2
mov bx, ax
pop ax
cwd            ; Convert word to double word
idiv bx
mov quot, ax

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Variable Reference

# Source: let y = x;
# Assembly:
mov ax, x
mov y, ax

# Source: print x;
# Assembly:
mov ax, x
call print_num
mov ah,09h
lea dx,msg
int 21h

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Complex Expression

# Source: let result = a + b * c;
# Assembly:
mov ax, b
push ax
mov ax, c
mov bx, ax
pop ax
imul bx        ; b * c
push ax
mov ax, a
mov bx, ax
pop ax
add ax, bx     ; a + (b * c)
mov result, ax

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Register Usage

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Primary Registers

  • AX: Accumulator - main computation register
  • BX: Base - used for right operand in binary operations
  • CX: Counter - used in print_num loop
  • DX: Data - used for I/O operations and division

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Stack Usage

The stack is used to save intermediate values in binary operations: 
; Evaluate left operand
mov ax, [left_value]
push ax              ; Save left value

; Evaluate right operand
mov ax, [right_value]
mov bx, ax

; Restore and compute
pop ax               ; Restore left value
add ax, bx           ; Perform operation
The generator includes a complete routine to print numbers: 
; imprimir numero en AX
print_num proc
push ax
push bx
push cx
push dx

mov cx,0
mov bx,10

cmp ax,0
jne convert

mov dl,'0'
mov ah,02h
int 21h
jmp done

convert:
next_digit:
xor dx,dx
div bx
push dx
inc cx
cmp ax,0
jne next_digit

print_loop:
pop dx
add dl,'0'
mov ah,02h
int 21h
loop print_loop

done:
pop dx
pop cx
pop bx
pop ax
ret
print_num endp
  1. Save registers to preserve values
  2. Handle zero special case
  3. Extract digits by dividing by 10 repeatedly
  4. Push digits onto stack (reverses order)
  5. Pop and print each digit as ASCII
  6. Restore registers and return

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Complete Example

from compfinal import Scanner, Parser, GeneradorASM

# Source code
code = """
let a = 5;
let b = 10;
let c = a + b * 2;
print c;
"""

# Compile to assembly
scanner = Scanner(code)
tokens = scanner.escanear_tokens()

parser = Parser(tokens)
programa = parser.parsear()

generador = GeneradorASM()
asm_code = generador.generar(programa)

# Save to file
with open('output.asm', 'w') as f:
    f.write(asm_code)

print("Assembly code generated: output.asm")
Generated output.asm: 
.model small
.stack 100h

.data
a dw 0
b dw 0
c dw 0
msg db 13,10,'$'

.code
main proc
mov ax, @data
mov ds, ax

mov ax, 5
mov a, ax

mov ax, 10
mov b, ax

mov ax, b
push ax
mov ax, 2
mov bx, ax
pop ax
imul bx
push ax
mov ax, a
mov bx, ax
pop ax
add ax, bx
mov c, ax

mov ax, c
call print_num
mov ah,09h
lea dx,msg
int 21h

mov ah,4ch
int 21h
main endp

; imprimir numero en AX
print_num proc
push ax
push bx
push cx
push dx
mov cx,0
mov bx,10
cmp ax,0
jne convert
mov dl,'0'
mov ah,02h
int 21h
jmp done
convert:
next_digit:
xor dx,dx
div bx
push dx
inc cx
cmp ax,0
jne next_digit
print_loop:
pop dx
add dl,'0'
mov ah,02h
int 21h
loop print_loop
done:
pop dx
pop cx
pop bx
pop ax
ret
print_num endp

end main

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DOS Interrupts Used

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INT 21h Services

  • AH=02h: Write character to standard output
  • AH=09h: Write string to standard output
  • AH=4Ch: Terminate program

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Arithmetic Instructions

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ADD

Addition: add ax, bx → AX = AX + BX

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SUB

Subtraction: sub ax, bx → AX = AX - BX

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IMUL

Signed multiplication: imul bx → AX = AX × BX

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IDIV

Signed division: 
cwd        ; Extend AX to DX:AX
idiv bx    ; AX = DX:AX ÷ BX, DX = remainder

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Implementation Details

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Expression Evaluation

Uses post-order traversal (evaluate children before parent): 
def generar_expr(self, expr):
    if isinstance(expr, NumeroLiteral):
        self.codigo.append(f"mov ax, {expr.valor}")
    
    elif isinstance(expr, Identificador):
        self.codigo.append(f"mov ax, {expr.nombre}")
    
    elif isinstance(expr, ExpresionBinaria):
        # 1. Evaluate left
        self.generar_expr(expr.izquierda)
        self.codigo.append("push ax")
        
        # 2. Evaluate right
        self.generar_expr(expr.derecha)
        self.codigo.append("mov bx, ax")
        
        # 3. Combine
        self.codigo.append("pop ax")
        # Apply operator...

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Limitations

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Integer Only

Only 16-bit signed integers are supported (-32768 to 32767).

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No Optimization

The code is not optimized:
  • Redundant moves are not eliminated
  • Constants are not folded
  • Dead code is not removed

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Basic I/O

Only integer output is supported via the print statement.

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EMU8086 Compatibility

The generated code is fully compatible with EMU8086 emulator:
  1. Save the generated code to a .asm file
  2. Open in EMU8086
  3. Compile and run
  4. View output in the emulator console

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See Also

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