listing.s

# This program draws 100 frames of noise to the screen and then stops.
# Using linux framebuffers.

# https://content.riscv.org/wp-content/uploads/2017/05/riscv-spec-v2.2.pdf
# https://en.wikichip.org/wiki/risc-v/registers
# https://rv8.io/syscalls.html
.section .text
.globl _start # stop ld from nagging.
.org 0
load_address = 0x010000 # can also use: 0x08048000
ehdr:
    .byte 0x7f, 0x45, 0x4c, 0x46, 2, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0
    .half 2                                 #   e_type
    .half 0xf3                              #   e_machine
    .word 1                                 #   e_version
    .quad load_address + _start - ehdr      #   e_entry
    .quad phdr - ehdr                       #   e_phoff
    .quad 0                                 #   e_shoff
    .word 0                                 #   e_flags
    .half ehdrsize                          #   e_ehsize    (64)
    .half phdrsize                          #   e_phentsize (56)
    .half 1                                 #   e_phnum
    .half 0                                 #   e_shentsize (64)
    .half 0                                 #   e_shnum
    .half 0                                 #   e_shstrndx
ehdrsize = . - ehdr

phdr:
    .word 1                                 #   p_type
    .word 5                                 #   p_flags
    .quad 0                                 #   p_offset
    .quad load_address                      #   p_vaddr
    .quad load_address                      #   p_paddr
    .quad filesize                          #   p_filesz
    .quad filesize                          #   p_memsz
    .quad 0x1000                            #   p_align
phdrsize = . - phdr

# Linux system call numbers we are using, https://rv8.io/syscalls.html
sys_ioctl = 29
sys_close = 57
sys_read  = 63
sys_write = 64
sys_exit  = 93
sys_brk     = 214
sys_munmmap = 215
sys_mmap = 222
sys_open = 1024
# standard file numbers
stdin  = 0
stdout = 1
stderr = 2
# Some file flags we need
O_RDWR = 0x0002
PROT_READ  = 0x1
PROT_WRITE = 0x2
PROT_EXEC = 0x4
PROT_READ_WRITE = 0x3
MAP_SHARED  = 0x1
MAP_PRIVATE = 0x2
MAP_FIXED = 0x10
MAP_ANON = 0x20
# Some framebuffer flags we need
FBIOGET_VSCREENINFO = 0x4600
FBIOGET_FSCREENINFO = 0x4602
GET_VSCREENINFO = 0x80A04600
GET_FSCREENINFO = 0x80504602

fb_var_screeninfo_size = 160
# struct fb_var_screeninfo {
# 	  0 __u32 xres;			/* visible resolution		*/
# 	  4 __u32 yres;
# 	  8 __u32 xres_virtual;		/* virtual resolution		*/
# 	 12 __u32 yres_virtual;
# 	 16 __u32 xoffset;			/* offset from virtual to visible */
# 	 20 __u32 yoffset;			/* resolution			*/
# 
# 	 24 __u32 bits_per_pixel;		/* guess what			*/
# 	 28 __u32 grayscale;		/* 0 = color, 1 = grayscale,	*/
# 					/* >1 = FOURCC			*/
# 	 32 struct fb_bitfield red;		/* bitfield in fb mem if true color, */
# 	 44 struct fb_bitfield green;	/* else only length is significant */
# 	 56 struct fb_bitfield blue;
# 	 68 struct fb_bitfield transp;	/* transparency			*/	
# 
# 	 80 __u32 nonstd;			/* != 0 Non standard pixel format */
# 
# 	 84 __u32 activate;			/* see FB_ACTIVATE_*		*/
# 
# 	 88 __u32 height;			/* height of picture in mm    */
# 	 92 __u32 width;			/* width of picture in mm     */
# 
# 	 96 __u32 accel_flags;		/* (OBSOLETE) see fb_info.flags */
# 
# 	/* Timing: All values in pixclocks, except pixclock (of course) */
# 	 100 __u32 pixclock;			/* pixel clock in ps (pico seconds) */
# 	 104 __u32 left_margin;		/* time from sync to picture	*/
# 	108 __u32 right_margin;		/* time from picture to sync	*/
# 	112 __u32 upper_margin;		/* time from sync to picture	*/
# 	116 __u32 lower_margin;
# 	120 __u32 hsync_len;		/* length of horizontal sync	*/
# 	124 __u32 vsync_len;		/* length of vertical sync	*/
# 	128 __u32 sync;			/* see FB_SYNC_*		*/
# 	132 __u32 vmode;			/* see FB_VMODE_*		*/
# 	136 __u32 rotate;			/* angle we rotate counter clockwise */
# 	140 __u32 colorspace;		/* colorspace for FOURCC-based modes */
# 	    __u32 reserved[4];		/* Reserved for future compatibility */

fb_bitfield_size = 12
# struct fb_bitfield {
# 	0 __u32 offset;			/* beginning of bitfield	*/
# 	4 __u32 length;			/* length of bitfield		*/
# 	8 __u32 msb_right;		/* != 0 : Most significant bit is */ 
# 					/* right */ 
# };

fb_fix_screeninfo_size = 80
# struct fb_fix_screeninfo {
# 	0 char id[16];			/* identification string eg "TT Builtin" */
# 	16 unsigned long smem_start;	/* Start of frame buffer mem */
# 					/* (physical address) */
# 	24 __u32 smem_len;			/* Length of frame buffer mem */
# 	28 __u32 type;			/* see FB_TYPE_*		*/
# 	32 __u32 type_aux;			/* Interleave for interleaved Planes */
# 	36 __u32 visual;			/* see FB_VISUAL_*		*/ 
# 	40 __u16 xpanstep;			/* zero if no hardware panning  */
# 	42 __u16 ypanstep;			/* zero if no hardware panning  */
# 	44 __u16 ywrapstep;		/* zero if no hardware ywrap    */
# 	48 __u32 line_length;		/* length of a line in bytes    */
# 	56 unsigned long mmio_start;	/* Start of Memory Mapped I/O   */
# 					/* (physical address) */
# 	64 __u32 mmio_len;			/* Length of Memory Mapped I/O  */
# 	68 __u32 accel;			/* Indicate to driver which	*/
# 					/*  specific chip/card we have	*/
# 	72 __u16 capabilities;		/* see FB_CAP_*			*/
# 	   __u16 reserved[2];		/* Reserved for future compatibility */
# };

_start:
    lla a0, fb0_path
    li  a1, O_RDWR
    li  a2, 0
    li  a7, sys_open
    ecall
    blt a0, zero, fail      # Fail if cannot open the file.
    mv gp, a0               # gp = fbfd

    addi sp, sp, -fb_fix_screeninfo_size
    mv a0, gp                           # fbfd
    li a1, FBIOGET_VSCREENINFO
    mv a2, sp                           # The screeninfo size pointer. 
    li a7, sys_ioctl
    ecall
    blt a0, zero, fail

    li  a0, stdout
    lla a1, greeting
    li  a2, 6
    li  a7, sys_write
    ecall

    addi sp, sp, -fb_var_screeninfo_size
    mv a0, gp                           # fbfd
    li a1, FBIOGET_VSCREENINFO
    mv a2, sp                           # The screeninfo size pointer. 
    li a7, sys_ioctl
    ecall
    blt a0, zero, fail

    # screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8
    lw a0, 0(sp) # xres
    lw a1, 4(sp) # yres
    lw a2, 24(sp) # bits_per_pixel
    mul a0, a0, a1 # t = xres * yres
    mul a0, a0, a2 # t = t * bits_per_pixel
    srl t0, a0, 3 # >> 8
    #t0 = screen size
    #li t0, 16588800

    li a0, 0
    mv a1, t0               # The screen size.
    li a2, PROT_READ_WRITE
    li a3, MAP_SHARED
    mv a4, gp               # fbfd
    li a5, 0                # offset
    li a7, sys_mmap
    ecall
    blt a0, zero, fail      # Fail if mapping failed.
    mv tp, a0               # tp = screenbuffer

    # seed for the random number generator.
    rdtime a0

    lcg_multiplier = 1664525
    lui a1, %hi(lcg_multiplier)
    addi a1, a1, %lo(lcg_multiplier)

    lcg_increment = 1013904223
    lui a2, %hi(lcg_increment)
    addi a2, a2, %lo(lcg_increment)

    li a7, 100 # do 100 times.
repeat_fill:
    blt a7, zero, repeat_done
    # the current pointer.
    li a3, 0
fill_screen_with_garbage:
    bge a3, t0, fill_screen_with_garbage_done

    mul a4, a0, a1             # seed = (a * seed + c) % m
    add a0, a4, a2

    srl a6, a0, 24
    add a5, tp, a3 # screenbuffer + index
    sb a6, 0(a5)
    # location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +
    #            (y+vinfo.yoffset) * finfo.line_length;

    addi a3, a3, 1
    j fill_screen_with_garbage

fill_screen_with_garbage_done:
    addi a7, a7, -1
    j repeat_fill
repeat_done:
    # li  a0, stdout
    # lla a1, greeting
    # li  a2, 6
    # li  a7, sys_write
    # ecall

    li a0, 0
    li a7, sys_exit
    ecall

fail:
    li a0, 1
    li a7, sys_exit
    ecall
    loop: j loop

greeting:
    .string "Hello\n"

fb0_path:
    .string "/dev/fb0"
    .byte 0

filesize = . - ehdr 

Makefile

# There was a bug in the riscv toolchain preventing use of --oformat binary.
# Use of objcopy is a workaround for it.
noise: noise.elf
	riscv64-unknown-elf-objcopy -O binary $^ $@ 
	chmod +x $@

noise.elf: listing.s
	riscv64-unknown-elf-gcc -nostartfiles $^ -o $@