216 lines
6.9 KiB
ArmAsm
216 lines
6.9 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
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cons_draw_line:
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movw $0, err # Make sure that err starts at 0
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movw %ax, (x0) # Move the initial vars from the registers to static memory
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movw %bx, (y0)
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movw %cx, (x1)
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movw %dx, (y1)
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movw %si, (color)
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movw x1, %cx # Load x1 and y1 into the registers
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movw y1, %dx
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sub x0, %cx # Remove x/y0 from x/y1 to get the delta values
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sub y0, %dx
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movw %cx, (deltax) # Store the delta values
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movw %dx, (deltay)
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cons_line_check_x:
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movw x0, %cx # Load x0 into register cx so we can manipulate this value to plot each pixel
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movw $1, (sx) # Preload the x slope with 1
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cmp x1, %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 # If x1 is greater than we need to flip the slope and the x delta values
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negw deltax # 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, %dx # Load y0 into register dx so we can manipulate this value to plot each pixel
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movw $1, (sy) # Preload the y slope with 1
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cmpw y1, %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 # if y1 is greater than we need to flip the slope y and delta y values
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negw deltay
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cons_line_prep_loop:
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movw deltax, %ax # Calculate the err variable by subtracting delta y from delta x
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sub deltay, %ax
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movw %ax, (err)
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cons_line_loop_start:
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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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movb $0x0c, %ah # Set the function byte to plot the pixel
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movb $0, %bh # Set the video page number
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movb (color), %al # Set the color of the pixel
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int $0x10 # Call the int
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cmpw x1, %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, %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, %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, %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 # Remove the deltay from the err check
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addw sx, %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, %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 # Add the deltax to the err value
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addw sy, %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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ret # 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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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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movw $50, %ax # Plot a line, ax = x0, bx = y0, cx = x1, dx = y1, si = color
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movw $50, %bx
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movw $160, %cx
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movw $55, %dx
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movw $12, %si # The Color using the si counter because we dont have enough normal registers
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call cons_draw_line
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# Draw the rest of the lines
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movw $160, %ax
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movw $55, %bx
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movw $90, %cx
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movw $150, %dx
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movw $11, %si
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call cons_draw_line
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movw $90, %ax
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movw $150, %bx
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movw $50, %cx
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movw $50, %dx
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movw $14, %si
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call cons_draw_line
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movw $10, %ax
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movw $10, %bx
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movw $310, %cx
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movw $10, %dx
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movw $9, %si
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call cons_draw_line
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movw $310, %ax
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movw $10, %bx
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movw $310, %cx
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movw $190, %dx
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movw $10, %si
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call cons_draw_line
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movw $310, %ax
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movw $190, %bx
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movw $10, %cx
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movw $190, %dx
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movw $13, %si
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call cons_draw_line
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movw $10, %ax
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movw $190, %bx
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movw $10, %cx
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movw $10, %dx
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movw $15, %si
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call cons_draw_line
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movw $75, %ax
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movw $150, %bx
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movw $250, %cx
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movw $110, %dx
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movw $1, %si
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call cons_draw_line
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movw $210, %ax
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movw $30, %bx
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movw $300, %cx
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movw $150, %dx
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movw $6, %si
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call cons_draw_line
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movw $180, %ax
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movw $180, %bx
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movw $170, %cx
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movw $20, %dx
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movw $3, %si
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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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x0: .word 0 # Setup the static memory to hold our variables
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y0: .word 0
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x1: .word 0
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y1: .word 0
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deltax: .word 0
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deltay: .word 0
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sx: .word 0
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sy: .word 0
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err: .word 0
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e2: .word 0
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color: .word 0 |