# 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 # 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, pixely, pixelColor) #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 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 real_start: movw $boot_message, %si # Display our boot message call cons_writeline movw $0xA000, %bx # Set the start of the video memory location movw %bx, %es # Move that address into the "extra segment" es register draw_start: # Set the Video mode to VGA 320 x 200 x 256 movb $0, %ah movb $0x13, %al int $0x10 xor %ax, %ax # Plot a line, we add the parameters to the stack in reverse order pushw $12 # Color pushw $55 # y1 pushw $160 # x1 pushw $50 # y0 pushw $50 # x0 call cons_draw_line # Draw the rest of the lines pushw $11 # Color pushw $150 # y1 pushw $90 # x1 pushw $55 # y0 pushw $160 # x0 call cons_draw_line pushw $14 # Color pushw $50 # y1 pushw $50 # x1 pushw $150 # y0 pushw $90 # x0 call cons_draw_line 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 pushw $1 # Color pushw $110 # y1 pushw $250 # x1 pushw $150 # y0 pushw $75 # x0 call cons_draw_line pushw $6 # Color pushw $150 # y1 pushw $300 # x1 pushw $30 # y0 pushw $210 # x0 call cons_draw_line pushw $3 # Color pushw $20 # y1 pushw $170 # x1 pushw $180 # y0 pushw $180 # x0 call cons_draw_line #--------------------------- pushw $11 # Color pushw $160 # y1 pushw $30 # x1 pushw $160 # y0 pushw $20 # x0 call cons_draw_line pushw $11 # Color pushw $180 # y1 pushw $30 # x1 pushw $160 # y0 pushw $30 # x0 call cons_draw_line pushw $11 # Color pushw $170 # y1 pushw $30 # x1 pushw $170 # y0 pushw $20 # x0 call cons_draw_line pushw $11 # Color pushw $180 # y1 pushw $30 # x1 pushw $180 # y0 pushw $20 # 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: # Global variables for the screen height and width .word 320 screen_height: .Word 200