basic physical memory page allocator

Signed-off-by: Matthew Wozniak <me@woz.blue>

1 files changed, 41 insertions, 0 deletions

diff --git a/src/mem.zig b/src/mem.zig
new file mode 100644
index 0000000..b361b88
--- /dev/null
+++ b/src/mem.zig
@@ -0,0 +1,41 @@
+const std = @import("std");
+
+const Page = extern struct {
+    next: *Page,
+};
+
+var head = @extern(*align(4096) Page, .{ .name = "_heap_start" });
+
+pub fn init() void {
+    // how many pages do we have?
+    const page_count = @intFromPtr(@extern(
+        *const anyopaque,
+        .{ .name = "_heap_size" },
+    )) / 4096;
+
+    // go through every page and make it point it to the next one
+    const pages: []Page = @as([*]Page, @ptrCast(head))[0..page_count];
+    for (0..page_count - 1) |i| {
+        pages[i].next = &pages[i + 1];
+    }
+
+    std.log.scoped(.mem).info(
+        "{} physical pages of 4096 B = {} B free",
+        .{ page_count, page_count * 4096 },
+    );
+}
+
+// allocates 1 page by taking it off the front of the linked list
+pub fn alloc() []align(4096) u8 {
+    const page = @as([*]align(4096) u8, @ptrCast(head))[0..4096];
+    head = head.next;
+    return page;
+}
+
+// free the page by putting it back on the front of the linked list
+pub fn free(mem: []align(4096) u8) void {
+    std.debug.assert(mem.len == 4096);
+    const page: *Page = @ptrCast(mem.ptr);
+    page.next = head;
+    head = page;
+}