# Best to keep these next few lines all together
#
# Settings for “Bitmap Display” given the following .eqv directives…
#
# Unit width in pixels: 32; unit height in pixels: 32
# Display width in pixels: 512; display height in pixels: 512
# Base address for display: 0x10010000
.eqv WIDTH_IN_PIXELS 16
.eqv WIDTH_IN_SHIFTS 4
.eqv HEIGHT_IN_PIXELS 16
.eqv HEIGHT_IN_SHIFTS 4
.eqv PIXEL_WIDTH 1
.eqv PIXEL_WIDTH_IN_SHIFTS 2
.eqv PIXEL_HEIGHT 1
.eqv PIXEL_HEIGHT_IN_SHIFTS 0
.eqv FRAME_BUFFER_SIZE 1024 # 16 * 16 * 4 (that is, one word per pixel)
.data 0x10014000 # room for activation frames in this file
SET_PIXEL_SAVES:
.space 16
RENDER_FRAME_SAVES:
.space 12
.data 0x10010000 # All pixel operations go directly to area mapped by Bitmap tool
FRAME_BUFFER:
.space FRAME_BUFFER_SIZE
.data
.text
# Assumption here is that thie file is “included” into some other file
# and we want control to begin in that other file’s own code. We
# assume, therefore, that the other file begins the program
# at “main”.
j main
# Should never, ever, **ever** return to this spoint in the code.
# $a0: row (counting from the top as row 0)
# $a1: column (counting from the left as column 0)
# $a2: colour (24-bit RGB value)
#
# Note that this code does *not* use the stack to save and
# restore register. This is pretty much a consequence of
# paranoia — that is, by separating the following code
# from any other routines that use the stack, we isolate
# the following code from any bugs in stack-maintenance
# operations.
#
set_pixel:
la $t1, SET_PIXEL_SAVES
sw $ra, 0($t1)
sw $s0, 4($t1)
sw $s1, 8($t1)
sw $s2, 12($t1)
# sanity check: 0 <= row < HEIGHT_IN_PIXELS? 0 <= col < WIDTH_IN_PIXELS? bltz $a0, set_pixel_exit bge $a0, HEIGHT_IN_PIXELS, set_pixel_exit bltz $a1, set_pixel_exit bge $a1, WIDTH_IN_PIXELS, set_pixel_exit # Need byte address of row add $s0, $zero, $a0 sll $s0, $s0, PIXEL_WIDTH_IN_SHIFTS sll $s0, $s0, PIXEL_HEIGHT_IN_SHIFTS sll $s0, $s0, WIDTH_IN_SHIFTS # Now need offset for column in that row addi $s1, $zero, 1 sll $s1, $s1, PIXEL_WIDTH_IN_SHIFTS sll $s1, $s1, PIXEL_HEIGHT_IN_SHIFTS mult $s1, $a1 mflo $s1 # Finally, compute the address of the first byte # in that pixel. la $s2, FRAME_BUFFER add $s2, $s2, $s0 add $s2, $s2, $s1 addi $s0, $zero, PIXEL_HEIGHT set_pixel_row: addi $s1, $zero, PIXEL_WIDTH set_pixel_column: sw $a2, 0($s2) addi $s2, $s2, 4 addi $s1, $s1, -1 bne $s1, $zero, set_pixel_column addi $s1, $zero, 1 sll $s1, $s1, PIXEL_WIDTH_IN_SHIFTS sll $s1, $s1, WIDTH_IN_SHIFTS add $s2, $s2, $s1 add $t0, $zero, PIXEL_WIDTH # wee tweak sub $s2, $s2, $t0 addi $s0, $s0, -1 bne $s0, $zero, set_pixel_row set_pixel_exit: la $t1, SET_PIXEL_SAVES lw $ra, 0($t1) lw $s0, 4($t1) lw $s1, 8($t1) lw $s2, 12($t1) jr $ra