feat: add multiboot2 framebuffer and Forth pixel primitives

Co-authored-by: aider (openrouter/moonshotai/kimi-k2.6) <aider@aider.chat>

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;
     }
 }
 
@@ -149,7 +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);
 }
@@ -239,9 +314,32 @@ 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);
 
+    struct mboot_tag* tag = (struct mboot_tag*)(mboot_ptr + 8);
+    while (tag->type != 0) {
+        if (tag->type == 5) {
+            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);
@@ -260,3 +358,36 @@ 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);
+}