329 lines
11 KiB
ArmAsm
329 lines
11 KiB
ArmAsm
# Second stage of the boot loader
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.code16 # Assemble for 16-bit mode
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.globl start
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start:
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jmp real_start
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# Write to the console using BIOS.
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#
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# Input: SI contains the address of the null-terminated string to be displayed
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cons_write:
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movb $0x0e, %ah # 0x0e is the INT 10h BIOS call to output the value contained in AL to screen
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cons_write_rpt:
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movb (%si), %al # Load the byte at the location contained in the SI register into AL
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inc %si # Add 1 to the value in SI
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cmp $0, %al # Compare the value in AL with 0
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jz cons_write_done # If it is zero, then we are done
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int $0x10 # Output the character in AL to the screen
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jmp cons_write_rpt # and continue
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cons_write_done: # Something that is called will never return
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ret # until a 'ret' instruction is encountered. Labels do
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# not give a program any structure. They just give a
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# memory location a name that we can use in our code.
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cons_write_crlf:
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movb $0x0e, %ah # Output CR
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movb $0x0d, %al
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int $0x10
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movb $0x0a, %al # Output LF
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int $0x10
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ret
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cons_writeline:
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call cons_write
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call cons_write_crlf
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ret
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# Draw Line function DrawLine(x0 (4), y0 (6), x1 (8), y1 (10), color (12))
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# Define parameter address positions in stack
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#define color 12
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#define y1 10
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#define x1 8
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#define y0 6
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#define x0 4
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# Define local variable positions in stack
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#define deltax -2
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#define deltay -4
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#define sx -6
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#define sy -8
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#define err -10
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#define e2 -12
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cons_draw_line:
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pushw %bp
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movw %sp, %bp
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subw $12, %sp # Make room for our local variables in the stack
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# Store existing register values to the stack so we can restore later
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pushw %ax
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pushw %bx
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pushw %cx
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pushw %dx
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pushw %si
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pushw %di
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movw $0, err(%bp) # Make sure that err starts at 0
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movw x1(%bp), %cx # Load x1 and y1 into the registers
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movw y1(%bp), %dx
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sub x0(%bp), %cx # Remove x/y0 from x/y1 to get the delta values
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sub y0(%bp), %dx
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movw %cx, deltax(%bp) # Store the delta values
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movw %dx, deltay(%bp)
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cons_line_check_x:
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movw x0(%bp), %cx # Load x0 into register cx so we can manipulate this value to plot each pixel
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movw $1, sx(%bp) # Preload the x slope with 1
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cmp x1(%bp), %cx # Check if x0 is less than x1 and we can move to y
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jl cons_line_check_y
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negw sx(%bp) # If x1 is greater than we need to flip the slope and the x delta values
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negw deltax(%bp) # Flipping the deltax here saves us from having to have an abs function because if
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# x0 was greater than x1 we know we have a negative delta value and we flip it to pos
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cons_line_check_y:
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movw y0(%bp), %dx # Load y0 into register dx so we can manipulate this value to plot each pixel
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movw $1, sy(%bp) # Preload the y slope with 1
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cmpw y1(%bp), %dx # Check if y0 is less than y1 and we can start our plotting
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jl cons_line_prep_loop
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negw sy(%bp) # if y1 is greater than we need to flip the slope y and delta y values
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negw deltay(%bp)
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cons_line_prep_loop:
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movw deltax(%bp), %ax # Calculate the err variable by subtracting delta y from delta x
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sub deltay(%bp), %ax
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movw %ax, err(%bp)
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cons_line_loop_start:
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pushw color(%bp)
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pushw %dx
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pushw %cx
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call cons_plot_pixel
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cmpw x1(%bp), %cx # Check if x0 and x1 are equal, if not then we are still plotting
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jne cons_line_loop_next_point
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cmpw y1(%bp), %dx # Check if y0 and y1 are equal, if not then we are still plotting
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jne cons_line_loop_next_point
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jmp cons_line_loop_end # if both x's and y's are equal then we can end the function
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cons_line_loop_next_point:
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movw err(%bp), %ax # Load err into ax so that we can change it
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sal %ax # e2 is 2 * err, so we can arithmatic shift left
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cons_line_loop_move_y_point:
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movw deltay(%bp), %bx
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negw %bx # We need negative deltay to compare
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cmpw %bx, %ax # Check if we need to apply the slope value to y
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jle cons_line_loop_move_x_point
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negw %bx # Change deltay back to normal
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subw %bx, err(%bp) # Remove the deltay from the err check
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addw sx(%bp), %cx # Add the slope value to the current y value
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cons_line_loop_move_x_point:
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movw deltax(%bp), %bx
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cmpw %bx, %ax # Check if we need to apply the x slope value
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jge cons_line_loop_start
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addw %bx, err(%bp) # Add the deltax to the err value
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addw sy(%bp), %dx # Add the slope value to the current x value
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jmp cons_line_loop_start # Go back to the start of the loop
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cons_line_loop_end:
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# Return all the original values to each register before we return back
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popw %di
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popw %si
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popw %dx
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popw %cx
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popw %bx
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popw %ax
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movw %bp, %sp
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popw %bp
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ret $10 # Finish the loop and return to the call address
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# we also tell it to free the 10 bytes in the stack for the paramters
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# 5 x Word (2 bytes)
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# Function PlotPixel(pixelx, pixely, pixelColor)
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#define pixelcolor 8
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#define pixely 6
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#define pixelx 4
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# I split the pixel plotting off into its own function so that we can use it for any other function that plots pixels
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# and any boundary checks will be applied with the same rules
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cons_plot_pixel:
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# Setup the stack
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pushw %bp
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movw %sp, %bp
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# Store existing register values to the stack so we can restore later
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pushw %ax
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pushw %bx
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pushw %cx
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pushw %dx
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pushw %si
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pushw %di
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xor %ax, %ax # Clear ax and bx for use with the draw function
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xor %bx, %bx
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movw pixelx(%bp), %cx # Move x and y into their registers
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movw pixely(%bp), %dx
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cmpw (screen_width), %cx # Check if the x value has gone past the width of the screen
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jg cons_plot_pixel_end # If so we ignore the pixel so that we dont draw into unrelated memory
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cmpw $0, %cx # also check if x has gotten less than 0
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jl cons_plot_pixel_end
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cmpw (screen_height), %dx # Do the same checks for the y position, i chose to ignore the pixel rather than
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jg cons_plot_pixel_end # end the entire draw because when we come to the circles and polygons we
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cmpw $0, %dx # can still partially show the output that falls within the boundaries
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jl cons_plot_pixel_end
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# Pixel point = 0xA0000 + (y * 320) + x
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movw (screen_width), %ax # Set ax to 320 so that we can multiply this by y
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mul %dx # does the (y * 320) part of our math
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add %cx, %ax # Add the value of x to register ax
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movw %ax, %si # Move the value of ax into the si counter
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movw pixelcolor(%bp), %bx # Load the color into a register
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movb %bl, %es:(%si) # Load the lower half of the color (since they should only be from 0 to 255)
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# and place it at the given byte in the segment
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cons_plot_pixel_end:
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# Return all the original values to each register before we return back
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popw %di
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popw %si
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popw %dx
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popw %cx
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popw %bx
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popw %ax
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movw %bp, %sp
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popw %bp
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ret $6 # Finish the loop and return to the call address
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real_start:
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movw $boot_message, %si # Display our boot message
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call cons_writeline
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movw $0xA000, %bx # Set the start of the video memory location
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movw %bx, %es # Move that address into the "extra segment" es register
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draw_start:
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# Set the Video mode to VGA 320 x 200 x 256
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movb $0, %ah
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movb $0x13, %al
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int $0x10
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xor %ax, %ax
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# Plot a line, we add the parameters to the stack in reverse order
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pushw $12 # Color
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pushw $55 # y1
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pushw $160 # x1
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pushw $50 # y0
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pushw $50 # x0
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call cons_draw_line
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# Draw the rest of the lines
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pushw $11 # Color
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pushw $150 # y1
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pushw $90 # x1
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pushw $55 # y0
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pushw $160 # x0
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call cons_draw_line
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pushw $14 # Color
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pushw $50 # y1
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pushw $50 # x1
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pushw $150 # y0
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pushw $90 # x0
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call cons_draw_line
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pushw $9 # Color
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pushw $10 # y1
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pushw $310 # x1
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pushw $10 # y0
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pushw $10 # x0
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call cons_draw_line
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pushw $10 # Color
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pushw $190 # y1
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pushw $310 # x1
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pushw $10 # y0
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pushw $310 # x0
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call cons_draw_line
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pushw $13 # Color
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pushw $190 # y1
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pushw $10 # x1
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pushw $190 # y0
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pushw $310 # x0
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call cons_draw_line
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pushw $15 # Color
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pushw $10 # y1
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pushw $10 # x1
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pushw $190 # y0
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pushw $10 # x0
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call cons_draw_line
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pushw $1 # Color
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pushw $110 # y1
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pushw $250 # x1
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pushw $150 # y0
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pushw $75 # x0
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call cons_draw_line
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pushw $6 # Color
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pushw $150 # y1
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pushw $300 # x1
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pushw $30 # y0
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pushw $210 # x0
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call cons_draw_line
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pushw $3 # Color
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pushw $20 # y1
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pushw $170 # x1
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pushw $180 # y0
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pushw $180 # x0
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call cons_draw_line
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#---------------------------
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pushw $11 # Color
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pushw $160 # y1
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pushw $30 # x1
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pushw $160 # y0
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pushw $20 # x0
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call cons_draw_line
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pushw $11 # Color
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pushw $180 # y1
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pushw $30 # x1
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pushw $160 # y0
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pushw $30 # x0
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call cons_draw_line
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pushw $11 # Color
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pushw $170 # y1
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pushw $30 # x1
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pushw $170 # y0
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pushw $20 # x0
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call cons_draw_line
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pushw $11 # Color
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pushw $180 # y1
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pushw $30 # x1
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pushw $180 # y0
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pushw $20 # x0
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call cons_draw_line
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endless_loop: # Loop forever more
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jmp endless_loop
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# Program data
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boot_message:
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.string "Boot Loader Stage 2 loaded"
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screen_width: # Global variables for the screen height and width
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.word 320
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screen_height:
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.Word 200 |