added physical memory manager

src/main.zig

@@ -2,6 +2,7 @@ const std = @import("std");
 const danos = @import("danos");
 const arch = @import("arch");
 const console = @import("console.zig");
+const pmm = @import("pmm.zig");
 const BootInfo = danos.BootInfo;
 
 /// The calling convention used to enter the kernel. Pinned to SysV explicitly:
@@ -45,6 +46,21 @@ fn kmain(boot_info: *const BootInfo) noreturn {
     con.print("  mem regions: {d}\n", .{regions.len});
     con.print("  usable RAM : {d} MiB\n", .{usable_pages * danos.page_size / (1024 * 1024)});
 
+    // Bring up the physical frame allocator over that map, and prove it works:
+    // allocate three frames, then hand them back.
+    pmm.init(boot_info.memory_map);
+    const s = pmm.stats();
+    con.print("\ndanos: frame allocator online\n", .{});
+    con.print("  free frames: {d} ({d} MiB)\n", .{ s.free_frames, s.free_frames * danos.page_size / (1024 * 1024) });
+    const f0 = pmm.alloc();
+    const f1 = pmm.alloc();
+    const f2 = pmm.alloc();
+    con.print("  alloc x3   : 0x{x} 0x{x} 0x{x}\n", .{ f0 orelse 0, f1 orelse 0, f2 orelse 0 });
+    if (f0) |p| pmm.free(p);
+    if (f1) |p| pmm.free(p);
+    if (f2) |p| pmm.free(p);
+    con.print("  after free : {d} frames free\n", .{pmm.stats().free_frames});
+
     con.write("\nkernel initialised; nothing left to do, halting.\n");
 
     arch.halt();