fix: use correct multiboot2 framebuffer info tag type (8 not 5)

Co-authored-by: aider (openrouter/anthropic/claude-opus-4.7) <aider@aider.chat>

Makefile

@@ -3,7 +3,7 @@ CFLAGS = -Wall -Wextra -g -std=c11
 COMMON_SRCS = forth_core.c forth_dict.c forth_words.c forth_interp.c main.c
 
 LINUX_SRCS = $(COMMON_SRCS) platform_linux.c
-KERNEL_SRCS = $(COMMON_SRCS) platform_kernel.c kernel_entry.S
+KERNEL_SRCS = $(COMMON_SRCS) platform_kernel.c kernel_entry.S font8x8.c
 KERNEL_LD = kernel.ld
 
 TARGET = forth

font8x8.c

@@ -0,0 +1,100 @@
+#include <stdint.h>
+
+const uint8_t font8x8[96][8] = {
+    {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00}, /* 32 ' ' */
+    {0x18,0x18,0x18,0x18,0x18,0x00,0x18,0x00}, /* 33 '!' */
+    {0x66,0x66,0x66,0x00,0x00,0x00,0x00,0x00}, /* 34 '"' */
+    {0x66,0x66,0xFF,0x66,0xFF,0x66,0x66,0x00}, /* 35 '#' */
+    {0x18,0x3E,0x60,0x3C,0x06,0x7C,0x18,0x00}, /* 36 '$' */
+    {0x62,0x66,0x0C,0x18,0x30,0x66,0x46,0x00}, /* 37 '%' */
+    {0x3C,0x66,0x3C,0x38,0x67,0x66,0x3F,0x00}, /* 38 '&' */
+    {0x06,0x0C,0x18,0x00,0x00,0x00,0x00,0x00}, /* 39 ''' */
+    {0x0C,0x18,0x30,0x30,0x30,0x18,0x0C,0x00}, /* 40 '(' */
+    {0x30,0x18,0x0C,0x0C,0x0C,0x18,0x30,0x00}, /* 41 ')' */
+    {0x00,0x66,0x3C,0xFF,0x3C,0x66,0x00,0x00}, /* 42 '*' */
+    {0x00,0x18,0x18,0x7E,0x18,0x18,0x00,0x00}, /* 43 '+' */
+    {0x00,0x00,0x00,0x00,0x00,0x18,0x18,0x30}, /* 44 ',' */
+    {0x00,0x00,0x00,0x7E,0x00,0x00,0x00,0x00}, /* 45 '-' */
+    {0x00,0x00,0x00,0x00,0x00,0x18,0x18,0x00}, /* 46 '.' */
+    {0x00,0x03,0x06,0x0C,0x18,0x30,0x60,0x00}, /* 47 '/' */
+    {0x3C,0x66,0x6E,0x76,0x66,0x66,0x3C,0x00}, /* 48 '0' */
+    {0x18,0x18,0x38,0x18,0x18,0x18,0x7E,0x00}, /* 49 '1' */
+    {0x3C,0x66,0x06,0x0C,0x30,0x60,0x7E,0x00}, /* 50 '2' */
+    {0x3C,0x66,0x06,0x1C,0x06,0x66,0x3C,0x00}, /* 51 '3' */
+    {0x06,0x0E,0x1E,0x66,0x7F,0x06,0x06,0x00}, /* 52 '4' */
+    {0x7E,0x60,0x7C,0x06,0x06,0x66,0x3C,0x00}, /* 53 '5' */
+    {0x3C,0x60,0x60,0x7C,0x66,0x66,0x3C,0x00}, /* 54 '6' */
+    {0x7E,0x06,0x0C,0x18,0x30,0x30,0x30,0x00}, /* 55 '7' */
+    {0x3C,0x66,0x66,0x3C,0x66,0x66,0x3C,0x00}, /* 56 '8' */
+    {0x3C,0x66,0x66,0x3E,0x06,0x06,0x3C,0x00}, /* 57 '9' */
+    {0x00,0x00,0x18,0x00,0x00,0x18,0x00,0x00}, /* 58 ':' */
+    {0x00,0x00,0x18,0x00,0x00,0x18,0x18,0x30}, /* 59 ';' */
+    {0x0C,0x18,0x30,0x60,0x30,0x18,0x0C,0x00}, /* 60 '<' */
+    {0x00,0x00,0x7E,0x00,0x7E,0x00,0x00,0x00}, /* 61 '=' */
+    {0x30,0x18,0x0C,0x06,0x0C,0x18,0x30,0x00}, /* 62 '>' */
+    {0x3C,0x66,0x06,0x0C,0x18,0x00,0x18,0x00}, /* 63 '?' */
+    {0x3C,0x66,0x6E,0x6E,0x60,0x62,0x3C,0x00}, /* 64 '@' */
+    {0x18,0x3C,0x66,0x7E,0x66,0x66,0x66,0x00}, /* 65 'A' */
+    {0x7C,0x66,0x66,0x7C,0x66,0x66,0x7C,0x00}, /* 66 'B' */
+    {0x3C,0x66,0x60,0x60,0x60,0x66,0x3C,0x00}, /* 67 'C' */
+    {0x78,0x6C,0x66,0x66,0x66,0x6C,0x78,0x00}, /* 68 'D' */
+    {0x7E,0x60,0x60,0x78,0x60,0x60,0x7E,0x00}, /* 69 'E' */
+    {0x7E,0x60,0x60,0x78,0x60,0x60,0x60,0x00}, /* 70 'F' */
+    {0x3C,0x66,0x60,0x6E,0x66,0x66,0x3C,0x00}, /* 71 'G' */
+    {0x66,0x66,0x66,0x7E,0x66,0x66,0x66,0x00}, /* 72 'H' */
+    {0x3C,0x18,0x18,0x18,0x18,0x18,0x3C,0x00}, /* 73 'I' */
+    {0x1E,0x0C,0x0C,0x0C,0x0C,0x6C,0x38,0x00}, /* 74 'J' */
+    {0x66,0x6C,0x78,0x70,0x78,0x6C,0x66,0x00}, /* 75 'K' */
+    {0x60,0x60,0x60,0x60,0x60,0x60,0x7E,0x00}, /* 76 'L' */
+    {0x63,0x77,0x7F,0x6B,0x63,0x63,0x63,0x00}, /* 77 'M' */
+    {0x66,0x76,0x7E,0x7E,0x6E,0x66,0x66,0x00}, /* 78 'N' */
+    {0x3C,0x66,0x66,0x66,0x66,0x66,0x3C,0x00}, /* 79 'O' */
+    {0x7C,0x66,0x66,0x7C,0x60,0x60,0x60,0x00}, /* 80 'P' */
+    {0x3C,0x66,0x66,0x66,0x66,0x3C,0x0E,0x00}, /* 81 'Q' */
+    {0x7C,0x66,0x66,0x7C,0x78,0x6C,0x66,0x00}, /* 82 'R' */
+    {0x3C,0x66,0x60,0x3C,0x06,0x66,0x3C,0x00}, /* 83 'S' */
+    {0x7E,0x18,0x18,0x18,0x18,0x18,0x18,0x00}, /* 84 'T' */
+    {0x66,0x66,0x66,0x66,0x66,0x66,0x3C,0x00}, /* 85 'U' */
+    {0x66,0x66,0x66,0x66,0x66,0x3C,0x18,0x00}, /* 86 'V' */
+    {0x63,0x63,0x63,0x6B,0x7F,0x77,0x63,0x00}, /* 87 'W' */
+    {0x66,0x66,0x3C,0x18,0x3C,0x66,0x66,0x00}, /* 88 'X' */
+    {0x66,0x66,0x66,0x3C,0x18,0x18,0x18,0x00}, /* 89 'Y' */
+    {0x7E,0x06,0x0C,0x18,0x30,0x60,0x7E,0x00}, /* 90 'Z' */
+    {0x3C,0x30,0x30,0x30,0x30,0x30,0x3C,0x00}, /* 91 '[' */
+    {0x00,0x60,0x30,0x18,0x0C,0x06,0x03,0x00}, /* 92 '\' */
+    {0x3C,0x0C,0x0C,0x0C,0x0C,0x0C,0x3C,0x00}, /* 93 ']' */
+    {0x18,0x3C,0x66,0x00,0x00,0x00,0x00,0x00}, /* 94 '^' */
+    {0x00,0x00,0x00,0x00,0x00,0x00,0x7E,0x00}, /* 95 '_' */
+    {0x18,0x18,0x0C,0x00,0x00,0x00,0x00,0x00}, /* 96 '`' */
+    {0x00,0x00,0x3C,0x06,0x3E,0x66,0x3E,0x00}, /* 97 'a' */
+    {0x00,0x60,0x60,0x7C,0x66,0x66,0x7C,0x00}, /* 98 'b' */
+    {0x00,0x00,0x3C,0x60,0x60,0x60,0x3C,0x00}, /* 99 'c' */
+    {0x00,0x06,0x06,0x3E,0x66,0x66,0x3E,0x00}, /* 100 'd' */
+    {0x00,0x00,0x3C,0x66,0x7E,0x60,0x3C,0x00}, /* 101 'e' */
+    {0x00,0x0E,0x18,0x3E,0x18,0x18,0x18,0x00}, /* 102 'f' */
+    {0x00,0x00,0x3E,0x66,0x66,0x3E,0x06,0x7C}, /* 103 'g' */
+    {0x00,0x60,0x60,0x7C,0x66,0x66,0x66,0x00}, /* 104 'h' */
+    {0x00,0x18,0x00,0x38,0x18,0x18,0x3C,0x00}, /* 105 'i' */
+    {0x00,0x06,0x00,0x06,0x06,0x06,0x06,0x3C}, /* 106 'j' */
+    {0x00,0x60,0x60,0x6C,0x78,0x6C,0x66,0x00}, /* 107 'k' */
+    {0x00,0x38,0x18,0x18,0x18,0x18,0x3C,0x00}, /* 108 'l' */
+    {0x00,0x00,0x66,0x7F,0x6B,0x63,0x63,0x00}, /* 109 'm' */
+    {0x00,0x00,0x7C,0x66,0x66,0x66,0x66,0x00}, /* 110 'n' */
+    {0x00,0x00,0x3C,0x66,0x66,0x66,0x3C,0x00}, /* 111 'o' */
+    {0x00,0x00,0x7C,0x66,0x66,0x7C,0x60,0x60}, /* 112 'p' */
+    {0x00,0x00,0x3E,0x66,0x66,0x3E,0x06,0x06}, /* 113 'q' */
+    {0x00,0x00,0x7C,0x66,0x60,0x60,0x60,0x00}, /* 114 'r' */
+    {0x00,0x00,0x3E,0x60,0x3C,0x06,0x7C,0x00}, /* 115 's' */
+    {0x00,0x18,0x18,0x7E,0x18,0x18,0x0C,0x00}, /* 116 't' */
+    {0x00,0x00,0x66,0x66,0x66,0x66,0x3E,0x00}, /* 117 'u' */
+    {0x00,0x00,0x66,0x66,0x66,0x3C,0x18,0x00}, /* 118 'v' */
+    {0x00,0x00,0x63,0x6B,0x7F,0x3E,0x36,0x00}, /* 119 'w' */
+    {0x00,0x00,0x66,0x3C,0x18,0x3C,0x66,0x00}, /* 120 'x' */
+    {0x00,0x00,0x66,0x66,0x66,0x3E,0x0C,0x78}, /* 121 'y' */
+    {0x00,0x00,0x7E,0x0C,0x18,0x30,0x7E,0x00}, /* 122 'z' */
+    {0x0C,0x18,0x18,0x30,0x18,0x18,0x0C,0x00}, /* 123 '{' */
+    {0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x00}, /* 124 '|' */
+    {0x30,0x18,0x18,0x0C,0x18,0x18,0x30,0x00}, /* 125 '}' */
+    {0x00,0x00,0x66,0x3C,0x00,0x00,0x00,0x00}, /* 126 '~' */
+    {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00}, /* 127 DEL */
+};

forth.h

@@ -204,6 +204,14 @@ void do_dot_quote(Word* w);
 void do_dot_quote_inner(Word* w);
 void do_words(Word* w);
 
+// Framebuffer
+void do_fb_addr(Word* w);
+void do_fb_width(Word* w);
+void do_fb_height(Word* w);
+void do_fb_pitch(Word* w);
+void do_fb_bpp(Word* w);
+void do_fb_plot(Word* w);
+
 // Memory
 void do_fetch(Word* w);
 void do_store(Word* w);

kernel_entry.S

@@ -8,6 +8,19 @@ multiboot_header:
     .long multiboot_header_end - multiboot_header
     .long -(0xe85250d6 + 0 + (multiboot_header_end - multiboot_header))
 
+    /* Framebuffer tag: type=5, size=20, width=height=0 (any), depth=32.
+     * Layout per multiboot2 spec:
+     *   u16 type, u16 flags, u32 size, u32 width, u32 height, u32 depth
+     * Padded to 8-byte alignment. */
+    .align 8
+    .word 5
+    .word 0
+    .long 20
+    .long 0
+    .long 0
+    .long 32
+    .long 0       /* padding to 8-byte alignment */
+
     .align 8
     .word 0
     .word 0
@@ -21,19 +34,24 @@ pml4:
 pdpt:
     .space 4096
 pd:
-    .space 4096
+    .space 4096 * 4
 
 .align 16
 stack_bottom:
     .space 16384
 stack_top:
 
+.align 8
+mboot_info_ptr:
+    .space 8
+
 .section .text
 .code32
 .global _start
 .type _start, @function
 _start:
     movl $stack_top, %esp
+    movl %ebx, mboot_info_ptr
 
     cld
     movl $__bss_start, %edi
@@ -46,13 +64,22 @@ _start:
     orl $0x03, %eax
     movl %eax, pml4
 
+    /* Populate 4 PDPT entries, each pointing to one of 4 PDs.
+     * This identity-maps the first 4 GiB using 2 MiB pages,
+     * which covers typical framebuffer/MMIO regions. */
     movl $pd, %eax
     orl $0x03, %eax
-    movl %eax, pdpt
+    movl %eax, pdpt + 0
+    addl $4096, %eax
+    movl %eax, pdpt + 8
+    addl $4096, %eax
+    movl %eax, pdpt + 16
+    addl $4096, %eax
+    movl %eax, pdpt + 24
 
     movl $0x00000083, %eax
     movl $pd, %edi
-    movl $512, %ecx
+    movl $2048, %ecx
 1:
     movl %eax, (%edi)
     addl $0x200000, %eax
@@ -80,9 +107,23 @@ _start:
 
 .code64
 long_mode_start:
+    /* Load data segment registers with the data descriptor (0x10).
+     * In long mode most of these are ignored for access checks, but
+     * %ss must be a valid (or null) selector so that iretq from an
+     * interrupt can safely reload it. */
+    movw $0x10, %ax
+    movw %ax, %ds
+    movw %ax, %es
+    movw %ax, %fs
+    movw %ax, %gs
+    movw %ax, %ss
+
     movabsq $stack_top, %rax
     movq %rax, %rsp
+    /* Ensure 16-byte alignment for the System V AMD64 ABI. */
+    andq $-16, %rsp
 
+    movq mboot_info_ptr, %rdi
     call kernel_main
     cli
 1:
@@ -93,6 +134,7 @@ long_mode_start:
 .global isr_keyboard
 .align 16
 isr_keyboard:
+    /* Save all caller-saved registers used by the SysV AMD64 ABI. */
     pushq %rax
     pushq %rcx
     pushq %rdx
@@ -102,8 +144,14 @@ isr_keyboard:
     pushq %r9
     pushq %r10
     pushq %r11
+    /* On interrupt entry the CPU pushed 5 qwords (40 bytes), and we
+     * pushed 9 more (72 bytes) = 112 bytes total. That leaves %rsp
+     * misaligned by 8 relative to a 16-byte boundary after the
+     * upcoming `call`. Push one dummy qword to realign. */
+    subq $8, %rsp
     cld
     call keyboard_handler
+    addq $8, %rsp
     popq %r11
     popq %r10
     popq %r9
@@ -127,8 +175,10 @@ isr_keyboard:
 .section .rodata
 .align 8
 gdt64:
-    .quad 0x0000000000000000
-    .quad 0x00209A0000000000
+    .quad 0x0000000000000000         /* 0x00: null descriptor         */
+    .quad 0x00209A0000000000         /* 0x08: 64-bit code, DPL=0      */
+    .quad 0x0000920000000000         /* 0x10: data, writable, present */
+gdt64_end:
 gdt64_pointer:
-    .word gdt64_pointer - gdt64 - 1
+    .word gdt64_end - gdt64 - 1
     .quad gdt64

main.c

@@ -65,6 +65,14 @@ void forth_run(void) {
     add_primitive(".\"",   do_dot_quote, F_IMMEDIATE); // immediate
     add_primitive("words", do_words, 0);
 
+    // Framebuffer
+    add_primitive("fb-addr",  do_fb_addr, 0);
+    add_primitive("fb-width", do_fb_width, 0);
+    add_primitive("fb-height", do_fb_height, 0);
+    add_primitive("fb-pitch", do_fb_pitch, 0);
+    add_primitive("fb-bpp",   do_fb_bpp, 0);
+    add_primitive("fb-plot",  do_fb_plot, 0);
+
     // Memory
     add_primitive("@",  do_fetch, 0);
     add_primitive("!",  do_store, 0);

platform_kernel.c

@@ -2,27 +2,102 @@
 #include <stdint.h>
 #include <stdarg.h>
 
-// VGA text-mode buffer
-static uint16_t* vga_buffer = (uint16_t*)0xB8000;
-static int vga_row = 0;
-static int vga_col = 0;
+extern const uint8_t font8x8[96][8];
 
-static void vga_putchar(char c) {
-    if (c == '\n') {
-        vga_col = 0;
-        vga_row++;
-    } else {
-        int idx = vga_row * 80 + vga_col;
-        vga_buffer[idx] = (uint16_t)((0x0F << 8) | (unsigned char)c);
-        vga_col++;
-        if (vga_col >= 80) {
-            vga_col = 0;
-            vga_row++;
+struct mboot_tag {
+    uint32_t type;
+    uint32_t size;
+};
+
+struct mboot_tag_framebuffer {
+    uint32_t type;
+    uint32_t size;
+    uint64_t framebuffer_addr;
+    uint32_t framebuffer_pitch;
+    uint32_t framebuffer_width;
+    uint32_t framebuffer_height;
+    uint8_t framebuffer_bpp;
+    uint8_t framebuffer_type;
+    uint16_t reserved;
+};
+
+static uint64_t fb_addr = 0;
+static uint32_t fb_pitch = 0;
+static uint32_t fb_width = 0;
+static uint32_t fb_height = 0;
+static uint8_t fb_bpp = 0;
+static int fb_cols = 0;
+static int fb_rows = 0;
+static int fb_col = 0;
+static int fb_row = 0;
+
+static void fb_draw_pixel(int x, int y, uint32_t color) {
+    if (x < 0 || y < 0 || (uint32_t)x >= fb_width || (uint32_t)y >= fb_height) return;
+    if (!fb_addr) return;
+    if (fb_bpp == 32) {
+        uint32_t* p = (uint32_t*)(fb_addr + y * fb_pitch + x * 4);
+        *p = color;
+    } else if (fb_bpp == 24) {
+        uint8_t* p = (uint8_t*)(fb_addr + y * fb_pitch + x * 3);
+        p[0] = color & 0xFF;
+        p[1] = (color >> 8) & 0xFF;
+        p[2] = (color >> 16) & 0xFF;
+    }
+}
+
+static void fb_scroll(void) {
+    int line_height = 8;
+    uint32_t bytes_per_pixel = (fb_bpp + 7) / 8;
+    uint32_t copy_bytes = fb_width * bytes_per_pixel;
+    for (uint32_t y = 0; y < fb_height - line_height; y++) {
+        uint8_t* dst = (uint8_t*)(fb_addr + y * fb_pitch);
+        uint8_t* src = (uint8_t*)(fb_addr + (y + line_height) * fb_pitch);
+        for (uint32_t i = 0; i < copy_bytes; i++) {
+            dst[i] = src[i];
         }
     }
-    if (vga_row >= 25) {
-        vga_row = 0;
-        vga_col = 0;
+    for (uint32_t y = fb_height - line_height; y < fb_height; y++) {
+        uint8_t* row = (uint8_t*)(fb_addr + y * fb_pitch);
+        for (uint32_t i = 0; i < copy_bytes; i++) {
+            row[i] = 0;
+        }
+    }
+}
+
+static void fb_draw_char(char c) {
+    if (!fb_addr || fb_cols == 0) return;
+    if (c == '\n') {
+        fb_col = 0;
+        fb_row++;
+    } else if (c == '\r') {
+        fb_col = 0;
+    } else if (c == '\b') {
+        if (fb_col > 0) fb_col--;
+    } else {
+        unsigned char ch = (unsigned char)c;
+        if (ch < 32 || ch > 126) ch = '?';
+        const uint8_t* glyph = font8x8[ch - 32];
+        int start_x = fb_col * 8;
+        int start_y = fb_row * 8;
+        for (int dy = 0; dy < 8; dy++) {
+            uint8_t row = glyph[dy];
+            for (int dx = 0; dx < 8; dx++) {
+                if (row & (0x80 >> dx)) {
+                    fb_draw_pixel(start_x + dx, start_y + dy, 0xFFFFFF);
+                } else {
+                    fb_draw_pixel(start_x + dx, start_y + dy, 0x000000);
+                }
+            }
+        }
+        fb_col++;
+    }
+    if (fb_col >= fb_cols) {
+        fb_col = 0;
+        fb_row++;
+    }
+    if (fb_row >= fb_rows) {
+        fb_scroll();
+        fb_row = fb_rows - 1;
     }
 }
 
@@ -50,16 +125,37 @@ static void serial_init(void) {
     outb(0x3F8 + 4, 0x0B);
 }
 
-static uint64_t idt_entries[256][2];
+/* Each 64-bit IDT entry is 16 bytes = two uint64_t words.
+ *
+ * Word 0 layout:
+ *   [15: 0] offset[15:0]
+ *   [31:16] segment selector
+ *   [34:32] IST (0 = no IST)
+ *   [39:35] reserved (0)
+ *   [47:40] type+attr  (0x8E = present, DPL=0, 64-bit interrupt gate)
+ *   [63:48] offset[31:16]
+ *
+ * Word 1 layout:
+ *   [31: 0] offset[63:32]
+ *   [63:32] reserved (0)
+ */
+typedef struct {
+    uint64_t low;
+    uint64_t high;
+} __attribute__((packed)) IdtEntry;
+
+static IdtEntry idt_entries[256];
 
 static void idt_set_gate(int vector, uint64_t offset, uint16_t selector, uint8_t type_attr) {
-    uint64_t low = (offset & 0xFFFFULL) |
-                   ((uint64_t)selector << 16) |
-                   ((uint64_t)(type_attr & 0xFF) << 40) |
-                   ((offset & 0xFFFF0000ULL) << 32);
-    uint64_t high = offset >> 32;
-    idt_entries[vector][0] = low;
-    idt_entries[vector][1] = high;
+    uint64_t low =
+        (offset & 0x000000000000FFFFULL)          |   /* offset[15:0]  */
+        ((uint64_t)selector          << 16)        |   /* selector      */
+        /* bits 34:32 = IST=0, bits 39:35 = 0 */
+        ((uint64_t)type_attr         << 40)        |   /* type+attr     */
+        ((offset  & 0x00000000FFFF0000ULL) << 32); /* offset[31:16] */
+    uint64_t high = offset >> 32;                      /* offset[63:32] */
+    idt_entries[vector].low  = low;
+    idt_entries[vector].high = high;
 }
 
 static struct {
@@ -128,7 +224,7 @@ void keyboard_handler(void) {
 }
 
 void forth_putchar(char c) {
-    vga_putchar(c);
+    fb_draw_char(c);
     while ((inb(0x3F8 + 5) & 0x20) == 0) { }
     outb(0x3F8, (uint8_t)c);
 }
@@ -218,14 +314,41 @@ void forth_printf(const char* fmt, ...) {
     va_end(ap);
 }
 
-void kernel_main(void) {
+void kernel_main(uint64_t mboot_ptr) {
     extern void isr_keyboard(void);
 
+    /* Multiboot2 info tag types:
+     *   5 = BIOS boot device, 6 = memory map, 8 = framebuffer info, ...
+     * Note: in the *header* (request side), type 5 means "framebuffer
+     * request", but the bootloader's *response* tag uses type 8. */
+    struct mboot_tag* tag = (struct mboot_tag*)(mboot_ptr + 8);
+    while (tag->type != 0) {
+        if (tag->type == 8) {
+            struct mboot_tag_framebuffer* fb = (struct mboot_tag_framebuffer*)tag;
+            fb_addr = fb->framebuffer_addr;
+            fb_pitch = fb->framebuffer_pitch;
+            fb_width = fb->framebuffer_width;
+            fb_height = fb->framebuffer_height;
+            fb_bpp = fb->framebuffer_bpp;
+        }
+        tag = (struct mboot_tag*)((uint8_t*)tag + ((tag->size + 7) & ~7));
+    }
+
+    if (fb_addr && fb_bpp >= 24) {
+        fb_cols = fb_width / 8;
+        fb_rows = fb_height / 8;
+        for (uint32_t y = 0; y < fb_height; y++) {
+            for (uint32_t x = 0; x < fb_width; x++) {
+                fb_draw_pixel(x, y, 0);
+            }
+        }
+    }
+
     serial_init();
 
     idt_set_gate(0x21, (uint64_t)isr_keyboard, 0x08, 0x8E);
-    idt_ptr.limit = sizeof(idt_entries) - 1;
-    idt_ptr.base = (uint64_t)idt_entries;
+    idt_ptr.limit = (uint16_t)(sizeof(idt_entries) - 1);
+    idt_ptr.base  = (uint64_t)(uintptr_t)idt_entries;
     __asm__ volatile ("lidt %0" : : "m"(idt_ptr));
 
     pic_remap();
@@ -239,3 +362,36 @@ void kernel_main(void) {
     forth_run();
     forth_panic();
 }
+
+void do_fb_addr(Word* w) {
+    (void)w;
+    data_push((int64_t)fb_addr);
+}
+
+void do_fb_width(Word* w) {
+    (void)w;
+    data_push((int64_t)fb_width);
+}
+
+void do_fb_height(Word* w) {
+    (void)w;
+    data_push((int64_t)fb_height);
+}
+
+void do_fb_pitch(Word* w) {
+    (void)w;
+    data_push((int64_t)fb_pitch);
+}
+
+void do_fb_bpp(Word* w) {
+    (void)w;
+    data_push((int64_t)fb_bpp);
+}
+
+void do_fb_plot(Word* w) {
+    (void)w;
+    int64_t color = data_pop();
+    int64_t y = data_pop();
+    int64_t x = data_pop();
+    fb_draw_pixel((int)x, (int)y, (uint32_t)color);
+}

platform_linux.c

@@ -107,6 +107,13 @@ void forth_printf(const char* fmt, ...) {
     va_end(ap);
 }
 
+void do_fb_addr(Word* w) { (void)w; data_push(0); }
+void do_fb_width(Word* w) { (void)w; data_push(0); }
+void do_fb_height(Word* w) { (void)w; data_push(0); }
+void do_fb_pitch(Word* w) { (void)w; data_push(0); }
+void do_fb_bpp(Word* w) { (void)w; data_push(0); }
+void do_fb_plot(Word* w) { (void)w; data_pop(); data_pop(); data_pop(); }
+
 void _start(void) {
     forth_run();
     syscall1(60, 0);