# Use the built-in font to draw a grapheme to real screen.
#
# We need to do this in machine code because Mu doesn't have global variables
# yet (for the start of video memory).
#
# There are uncomfortable assumptions baked in here about english/latin
# script. We convert the grid of pixels into a fixed-width grid of graphemes,
# which may not work well with other language families.

== code

# The Mu computer's screen is 1024px wide and 768px tall.
# The Mu computer's font is 8px wide and 16px tall.
# Therefore 'x' here is in [0, 128), and 'y' is in [0, 48)
draw-grapheme-on-real-screen:  # g: grapheme, x: int, y: int, color: int, background-color: int
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    50/push-eax
    51/push-ecx
    52/push-edx
    53/push-ebx
    56/push-esi
    # var letter-bitmap/esi = font[g]
    8b/-> *(ebp+8) 6/r32/esi
    c1 4/subop/shift-left %esi 4/imm8
    8d/copy-address *(esi+0x8800) 6/r32/esi  # font-start
    # if (letter-bitmap >= 0x9000) return  # characters beyond ASCII currently not supported
    81 7/subop/compare %esi 0x9000/imm32
    7d/jump-if->= $draw-grapheme-on-real-screen:end/disp8
    # var ycurr/edx: int = y*16
    8b/-> *(ebp+0x10) 2/r32/edx
    c1 4/subop/shift-left %edx 4/imm8
    # var ymax/ebx: int = ycurr + 16
    8b/-> *(ebp+0x10) 3/r32/ebx
    c1 4/subop/shift-left %ebx 4/imm8
    81 0/subop/add %ebx 0x10/imm32
    {
      # if (ycurr >= ymax) break
      39/compare %edx 3/r32/ebx
      7d/jump-if->= break/disp8
      # var xcurr/eax: int = x*8 + 7
      8b/-> *(ebp+0xc) 0/r32/eax  # font-width - 1
      c1 4/subop/shift-left %eax 3/imm8
      81 0/subop/add %eax 7/imm32
      # var xmin/ecx: int = x*8
      8b/-> *(ebp+0xc) 1/r32/ecx
      c1 4/subop/shift-left %ecx 3/imm8
      # var row-bitmap/ebx: int = *letter-bitmap
      53/push-ebx
      8b/-> *esi 3/r32/ebx
      {
        # if (xcurr < xmin) break
        39/compare %eax 1/r32/ecx
        7c/jump-if-< break/disp8
        # shift LSB from row-bitmap into carry flag (CF)
        c1 5/subop/shift-right-logical %ebx 1/imm8
        # if LSB, draw a pixel in the given color
        {
          73/jump-if-not-CF break/disp8
          (pixel-on-real-screen %eax %edx *(ebp+0x14))
          eb/jump $draw-grapheme-on-real-screen:continue/disp8
        }
        # otherwise use the background color
        (pixel-on-real-screen %eax %edx *(ebp+0x18))
$draw-grapheme-on-real-screen:continue:
        # --x
        48/decrement-eax
        #
        eb/jump loop/disp8
      }
      # reclaim row-bitmap
      5b/pop-to-ebx
      # ++y
      42/increment-edx
      # next bitmap row
      46/increment-esi
      #
      eb/jump loop/disp8
    }
$draw-grapheme-on-real-screen:end:
    # . restore registers
    5e/pop-to-esi
    5b/pop-to-ebx
    5a/pop-to-edx
    59/pop-to-ecx
    58/pop-to-eax
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

cursor-position-on-real-screen:  # -> _/eax: int, _/ecx: int
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # TODO: support fake screen; we currently assume 'screen' is always 0 (real)
    8b/-> *Real-screen-cursor-x 0/r32/eax
    8b/-> *Real-screen-cursor-y 1/r32/ecx
$cursor-position-on-real-screen:end:
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

# Caller is responsible for tracking what was on the screen at this position
# before, and making sure the cursor continues to show the same grapheme.
set-cursor-position-on-real-screen:  # x: int, y: int, g: grapheme
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    50/push-eax
    #
    (draw-grapheme-on-real-screen *(ebp+0x10) *(ebp+8) *(ebp+0xc) 0 7)
    # TODO: support fake screen; we currently assume 'screen' is always 0 (real)
    8b/-> *(ebp+8) 0/r32/eax
    89/<- *Real-screen-cursor-x 0/r32/eax
    8b/-> *(ebp+0xc) 0/r32/eax
    89/<- *Real-screen-cursor-y 0/r32/eax
$set-cursor-position-on-real-screen:end:
    # . restore registers
    58/pop-to-eax
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

== data

# The cursor is where certain Mu functions (usually of the form
# 'draw*cursor*') print to by default. It becomes visible on
# set-cursor-position, but further drawing might overwrite it.
# We don't bother displaying the cursor when drawing text. It's up to
# applications to display the cursor before waiting for a key, and to ensure
# its location appropriately suggests the effect keystrokes will have.
#
# There's no low-level support for blinking, etc. We aren't using any
# hardware-supported text mode here.
Real-screen-cursor-x:
  0/imm32
Real-screen-cursor-y:
  0/imm32