# Second stage of the boot loader .code16 # Assemble for 16-bit mode .globl start start: jmp real_start # Write to the console using BIOS. # # Input: SI contains the address of the null-terminated string to be displayed cons_write: movb $0x0e, %ah # 0x0e is the INT 10h BIOS call to output the value contained in AL to screen cons_write_rpt: movb (%si), %al # Load the byte at the location contained in the SI register into AL inc %si # Add 1 to the value in SI cmp $0, %al # Compare the value in AL with 0 jz cons_write_done # If it is zero, then we are done int $0x10 # Output the character in AL to the screen jmp cons_write_rpt # and continue cons_write_done: # Something that is called will never return ret # until a 'ret' instruction is encountered. Labels do # not give a program any structure. They just give a # memory location a name that we can use in our code. cons_write_crlf: movb $0x0e, %ah # Output CR movb $0x0d, %al int $0x10 movb $0x0a, %al # Output LF int $0x10 ret cons_writeline: call cons_write call cons_write_crlf ret # Added Write Hex and Int functions to help with debugging HexChars: .ascii "0123456789ABCDEF" cons_write_hex: movw $4, %cx movb $0x0E, %ah hexloop: rol $4, %bx movw %bx, %si and $0x000F, %si movb HexChars(%si), %al int $0x10 loop hexloop ret cons_write_int: movw $IntBuffer + 4, %si movw %bx, %ax cmpw $0, %ax jge getdigit xor %ax, %ax # Added to handle signed numbers, it adds the minus and then neg's the number movb $0x0E, %ah movb $0x2D, %al int $0x10 movw %bx, %ax negw %ax getdigit: xor %dx, %dx movw $10, %cx idiv %cx addb $48, %dl movb %dl, (%si) dec %si cmp $0, %ax jne getdigit inc %si call cons_write ret IntBuffer: .string " " # Draw Line function DrawLine(x0 (4), y0 (6), x1 (8), y1 (10), color (12)) # Define parameter address positions in stack #define color 12 #define y1 10 #define x1 8 #define y0 6 #define x0 4 # Define local variable positions in stack #define deltax -2 #define deltay -4 #define sx -6 #define sy -8 #define err -10 #define e2 -12 cons_draw_line: pushw %bp movw %sp, %bp subw $12, %sp # Make room for our local variables in the stack # Store existing register values to the stack so we can restore later pushw %ax pushw %bx pushw %cx pushw %dx pushw %si pushw %di movw $0, err(%bp) # Make sure that err starts at 0 movw x1(%bp), %cx # Load x1 and y1 into the registers movw y1(%bp), %dx sub x0(%bp), %cx # Remove x/y0 from x/y1 to get the delta values sub y0(%bp), %dx movw %cx, deltax(%bp) # Store the delta values movw %dx, deltay(%bp) cons_line_check_x: movw x0(%bp), %cx # Load x0 into register cx so we can manipulate this value to plot each pixel movw $1, sx(%bp) # Preload the x slope with 1 cmp x1(%bp), %cx # Check if x0 is less than x1 and we can move to y jl cons_line_check_y negw sx(%bp) # If x1 is greater than we need to flip the slope and the x delta values negw deltax(%bp) # Flipping the deltax here saves us from having to have an abs function because if # x0 was greater than x1 we know we have a negative delta value and we flip it to pos cons_line_check_y: movw y0(%bp), %dx # Load y0 into register dx so we can manipulate this value to plot each pixel movw $1, sy(%bp) # Preload the y slope with 1 cmpw y1(%bp), %dx # Check if y0 is less than y1 and we can start our plotting jl cons_line_prep_loop negw sy(%bp) # if y1 is greater than we need to flip the slope y and delta y values negw deltay(%bp) cons_line_prep_loop: movw deltax(%bp), %ax # Calculate the err variable by subtracting delta y from delta x sub deltay(%bp), %ax movw %ax, err(%bp) cons_line_loop_start: pushw color(%bp) pushw %dx pushw %cx call cons_plot_pixel cmpw x1(%bp), %cx # Check if x0 and x1 are equal, if not then we are still plotting jne cons_line_loop_next_point cmpw y1(%bp), %dx # Check if y0 and y1 are equal, if not then we are still plotting jne cons_line_loop_next_point jmp cons_line_loop_end # if both x's and y's are equal then we can end the function cons_line_loop_next_point: movw err(%bp), %ax # Load err into ax so that we can change it sal %ax # e2 is 2 * err, so we can arithmatic shift left cons_line_loop_move_y_point: movw deltay(%bp), %bx negw %bx # We need negative deltay to compare cmpw %bx, %ax # Check if we need to apply the slope value to y jle cons_line_loop_move_x_point negw %bx # Change deltay back to normal subw %bx, err(%bp) # Remove the deltay from the err check addw sx(%bp), %cx # Add the slope value to the current y value cons_line_loop_move_x_point: movw deltax(%bp), %bx cmpw %bx, %ax # Check if we need to apply the x slope value jge cons_line_loop_start addw %bx, err(%bp) # Add the deltax to the err value addw sy(%bp), %dx # Add the slope value to the current x value jmp cons_line_loop_start # Go back to the start of the loop cons_line_loop_end: # Return all the original values to each register before we return back popw %di popw %si popw %dx popw %cx popw %bx popw %ax movw %bp, %sp popw %bp ret $10 # Finish the loop and return to the call address # we also tell it to free the 10 bytes in the stack for the paramters # 5 x Word (2 bytes) # Function PlotPixel(pixelx (4), pixely (6), pixelColor (8)) #define pixelcolor 8 #define pixely 6 #define pixelx 4 # I split the pixel plotting off into its own function so that we can use it for any other function that plots pixels # and any boundary checks will be applied with the same rules cons_plot_pixel: # Setup the stack pushw %bp movw %sp, %bp # Store existing register values to the stack so we can restore later pushw %ax pushw %bx pushw %cx pushw %dx pushw %si pushw %di xor %ax, %ax # Clear ax and bx for use with the draw function xor %bx, %bx movw pixelx(%bp), %cx # Move x and y into their registers movw pixely(%bp), %dx cmpw (screen_width), %cx # Check if the x value has gone past the width of the screen jg cons_plot_pixel_end # If so we ignore the pixel so that we dont draw into unrelated memory cmpw $0, %cx # also check if x has gotten less than 0 jl cons_plot_pixel_end cmpw (screen_height), %dx # Do the same checks for the y position, i chose to ignore the pixel rather than jg cons_plot_pixel_end # end the entire draw because when we come to the circles and polygons we cmpw $0, %dx # can still partially show the output that falls within the boundaries jl cons_plot_pixel_end # Pixel point = 0xA0000 + (y * 320) + x movw (screen_width), %ax # Set ax to 320 so that we can multiply this by y mul %dx # does the (y * 320) part of our math add %cx, %ax # Add the value of x to register ax movw %ax, %si # Move the value of ax into the si counter movw $0xA000, %bx # Set the start of the video memory location movw %bx, %es # Move that address into the "extra segment" es register movw pixelcolor(%bp), %bx # Load the color into a register movb %bl, %es:(%si) # Load the lower half of the color (since they should only be from 0 to 255) # and place it at the given byte in the segment cons_plot_pixel_end: # Return all the original values to each register before we return back popw %di popw %si popw %dx popw %cx popw %bx popw %ax movw %bp, %sp popw %bp ret $6 # Finish the loop and return to the call address # Draw Line function DrawFilledRect(rectx (4), recty (6), rectWidth (8), rectHeight (10), rectColor (12)) # Define parameter address positions in stack #define rectcolor 12 #define rectheight 10 #define rectwidth 8 #define recty 6 #define rectx 4 #define rectendx -2 #define rectendy -4 cons_draw_filled_rect: pushw %bp movw %sp, %bp subw $4, %sp # Make room for our local variables in the stack # Store existing register values to the stack so we can restore later pushw %ax pushw %bx pushw %cx pushw %dx pushw %si pushw %di cons_filled_rect_setup: movw rectwidth(%bp), %ax add rectx(%bp), %ax movw %ax, rectendx(%bp) movw rectheight(%bp), %bx add recty(%bp), %bx movw %bx, rectendy(%bp) movw $0xA000, %bx # Set the start of the video memory location movw %bx, %es # Move that address into the "extra segment" es register movw recty(%bp), %dx movw (screen_width), %ax # Set ax to 320 so that we can multiply this by y mul %dx # does the (y * 320) part of our math add rectx(%bp), %ax # Add the value of x to register ax movw %ax, %si lea %es:(%si), %bx # Move the value of ax into the si counter movw rectheight(%bp), %si cons_filled_rect_loop_start: movw rectcolor(%bp), %ax movw %bx, %di movw rectwidth(%bp), %cx cld rep stosb movw (screen_width), %cx add %cx, %bx dec %si jnz cons_filled_rect_loop_start cons_filled_rect_loop_end: # Return all the original values to each register before we return back popw %di popw %si popw %dx popw %cx popw %bx popw %ax movw %bp, %sp popw %bp ret $10 # Finish the loop and return to the call address # we also tell it to free the 10 bytes in the stack for the paramters # 5 x Word (2 bytes) # Draw Line function DrawCircle(circlex (4), circley (6), circleRadius (8), circleColor (10)) # This follows the bresenham circle drawing algorithm so that we can stick to integer values # Define parameter address positions in stack #define circlecolor 10 #define circleradius 8 #define circley 6 #define circlex 4 # Define local variable positions in stack #define circled -4 # Decision variable #define circlexcax -6 #define circlexcsx -8 #define circleycay -10 #define circleycsy -12 #define circlexcay -14 #define circlexcsy -16 #define circleycax -18 #define circleycsx -20 cons_draw_circle: pushw %bp movw %sp, %bp subw $20, %sp # Make room for our local variables in the stack # Store existing register values to the stack so we can restore later pushw %ax pushw %bx pushw %cx pushw %dx pushw %si pushw %di cons_draw_circle_setup: movw $0, %cx # x starts at 0 movw circleradius(%bp), %dx # y starts as the radius movw circleradius(%bp), %bx # d = 3 - (2 * r), we load the radius into a register sal %bx # and multiply it by 2 movw $3, %ax # load a 3 into a register to subtract the above from sub %bx, %ax movw %ax, circled(%bp) # Move the d variable into the stack cons_circle_loop_start: # We need to plot all 8 points for this step xor %ax, %ax # Clear a and b registers for use below xor %bx, %bx movw circlex(%bp), %ax # Points xc + x, yc + y add %cx, %ax movw circley(%bp), %bx add %dx, %bx movw %ax, circlexcax(%bp) movw %bx, circleycay(%bp) movw circlex(%bp), %ax # Points xc - x, yc - y sub %cx, %ax movw circley(%bp), %bx sub %dx, %bx movw %ax, circlexcsx(%bp) movw %bx, circleycsy(%bp) movw circlex(%bp), %ax # Points xc + y, yc + x add %dx, %ax movw circley(%bp), %bx add %cx, %bx movw %ax, circlexcay(%bp) movw %bx, circleycax(%bp) movw circlex(%bp), %ax # Points xc - y, yc - x sub %dx, %ax movw circley(%bp), %bx sub %cx, %bx movw %ax, circlexcsy(%bp) movw %bx, circleycsx(%bp) # Plot the 8 pixels for this turn of the circle pushw circlecolor(%bp) pushw circleycay(%bp) pushw circlexcax(%bp) call cons_plot_pixel pushw circlecolor(%bp) pushw circleycay(%bp) pushw circlexcsx(%bp) call cons_plot_pixel pushw circlecolor(%bp) pushw circleycsy(%bp) pushw circlexcax(%bp) call cons_plot_pixel pushw circlecolor(%bp) pushw circleycsy(%bp) pushw circlexcsx(%bp) call cons_plot_pixel pushw circlecolor(%bp) pushw circleycax(%bp) pushw circlexcay(%bp) call cons_plot_pixel pushw circlecolor(%bp) pushw circleycax(%bp) pushw circlexcsy(%bp) call cons_plot_pixel pushw circlecolor(%bp) pushw circleycsx(%bp) pushw circlexcay(%bp) call cons_plot_pixel pushw circlecolor(%bp) pushw circleycsx(%bp) pushw circlexcsy(%bp) call cons_plot_pixel inc %cx # Inc the x value cmpw $0, circled(%bp) # check if the decision variable is less than 0 jle cons_circle_skip_y dec %dx # If not we decrement the Y value and calculate the new decision value movw %cx, %ax # Move the x value into ax for mul movw %dx, %bx # Move the y value into bx since mul destroys the value in dx movw $4, %si # Move 4 into si because we are out of registers sub %bx, %ax # d = d + 4 * (x - y) + 10 imul %si add $10, %ax add %ax, circled(%bp) # Add the result to the current D value movw %bx, %dx # Move y back into the dx register jmp cons_circle_check_end # jump over the next section to the end check cons_circle_skip_y: movw %cx, %ax # If the decision var was greater than 0 we use another formula for d movw %dx, %bx # Store y in bx because we are using mul again movw $4, %si # d = d + 4 * x + 6 imul %si add $6, %ax add %ax, circled(%bp) movw %bx, %dx # Restore y to the dx register cons_circle_check_end: cmpw %cx, %dx # Check if y is greater than or equal to x jge cons_circle_loop_start # If so we carry on the loop until it is no longer cons_circle_loop_end: # Return all the original values to each register before we return back popw %di popw %si popw %dx popw %cx popw %bx popw %ax movw %bp, %sp popw %bp ret $8 # Finish the loop and return to the call address # we also tell it to free the 10 bytes in the stack for the paramters # 4 x Word (2 bytes) real_start: movw $boot_message, %si # Display our boot message call cons_writeline draw_start: # Set the Video mode to VGA 320 x 200 x 256 movb $0, %ah movb $0x13, %al int $0x10 xor %ax, %ax # Setup the registers to loop through the flag stripes movw $150, %ax # x Offset movw $15, %bx # y Offset movw $6, %cx # Sets of stripes (12 red/white) for the loop to decrement draw_flag_loop: # Draw our filled rectangles for the flag stripes add $5, %bx # add the height of stripe to the y offset pushw $12 # Color pushw $5 # height pushw $120 # width pushw %bx # y pushw %ax # x call cons_draw_filled_rect add $5, %bx # add the height of stripe to the y offset pushw $15 # Color pushw $5 # height pushw $120 # width pushw %bx # y pushw %ax # x call cons_draw_filled_rect loop draw_flag_loop draw_flag_loop_end: # Draw the 13th stripe add $5, %bx pushw $12 # Color pushw $5 # height pushw $120 # width pushw %bx # y pushw %ax # x call cons_draw_filled_rect # Draw the blue box that would hold the stars pushw $1 # Color pushw $35 # height pushw $45 # width pushw $20 # y pushw $150 # x call cons_draw_filled_rect pushw $6 # Color pushw $180 # height pushw $5 # width pushw $20 # y pushw $145 # x call cons_draw_filled_rect # Draw some circles to show off that function pushw $14 # Color pushw $3 # radius pushw $17 # y pushw $147 # x call cons_draw_circle pushw $2 # Color pushw $25 # radius pushw $80 # y pushw $90 # x call cons_draw_circle pushw $15 # Color pushw $5 # radius pushw $75 # y pushw $80 # x call cons_draw_circle pushw $15 # Color pushw $5 # radius pushw $75 # y pushw $100 # x call cons_draw_circle # Plot a line, we add the parameters to the stack in reverse order pushw $15 # Color pushw $90 # y1 pushw $75 # x1 pushw $95 # y0 pushw $90 # x0 call cons_draw_line pushw $15 # Color pushw $90 # y1 pushw $105 # x1 pushw $95 # y0 pushw $90 # x0 call cons_draw_line # Draw the rest of the lines pushw $2 # Color pushw $200 # y1 pushw $90 # x1 pushw $105 # y0 pushw $90 # x0 call cons_draw_line pushw $2 # Color pushw $100 # y1 pushw $145 # x1 pushw $120 # y0 pushw $90 # x0 call cons_draw_line # Line borders pushw $9 # Color pushw $10 # y1 pushw $310 # x1 pushw $10 # y0 pushw $10 # x0 call cons_draw_line pushw $10 # Color pushw $190 # y1 pushw $310 # x1 pushw $10 # y0 pushw $310 # x0 call cons_draw_line pushw $13 # Color pushw $190 # y1 pushw $10 # x1 pushw $190 # y0 pushw $310 # x0 call cons_draw_line pushw $15 # Color pushw $10 # y1 pushw $10 # x1 pushw $190 # y0 pushw $10 # x0 call cons_draw_line endless_loop: # Loop forever more jmp endless_loop # Program data boot_message: .string "Boot Loader Stage 2 loaded" screen_width: .word 320 screen_height: .word 200