Scanning for useful mem for heap

lboot.S

@@ -29,6 +29,10 @@ _start:     cli
             call        print
             call        getmap
 
+            mov         $msg_mkheap, %si
+            call        print
+            call        makeheap
+
             mov         $msg_paging, %si
             call        print
             call        paging
@@ -130,6 +134,51 @@ _pgnext:    mov         %ecx, %eax
 
             ret
 
+            // makeheap: find a memory range suitable for heap usage
+makeheap:   mov         $memmap-24, %si
+_mhnext:    add         $24, %si
+            cmp         memmap_end, %si
+            jae         _mhdone
+
+            cmpl        $1, 16(%si)
+            jne         _mhnext
+            
+            cmpl        $0, 4(%si)
+            ja          _mhnext
+            
+            mov         0(%si), %ebx
+            mov         8(%si), %ecx
+
+            // find end of range, clip to 4Gb
+            add         %ebx, %ecx
+            jnc         1f
+            mov         $0xFFFFFFFF, %ecx
+
+            // handle wraparound if length > 4Gb
+1:          cmpl        $0, 12(%si)
+            je          1f
+            mov         $0xFFFFFFFF, %ecx
+
+            // adjust base to above 1Mb, above the bootloader
+1:          cmp         $0x10000, %ebx
+            jae         1f
+            mov         $0x10000, %ebx
+
+            // align to 4Kb boundaries
+1:          add         $0xFFF, %ebx
+            and         $0xFFFFF000, %ebx
+            and         $0xFFFFF000, %ecx
+
+            sub         %ebx, %ecx
+            cmp         heap_size, %ecx
+            jbe         _mhnext
+
+            mov         %ebx, heap_start
+            mov         %ecx, heap_size
+            jmp         _mhnext
+
+_mhdone:    ret
+
             // alloc: take ECX bytes from usable space in memmap
             // No realignment is performed, so only alloc aligned sizes.
 alloc:      mov         $memmap-24, %si
@@ -203,10 +252,14 @@ msg_start:  .asciz      "Netboot via fernlader v1 ...\r\n"
 msg_a20:    .asciz      " * Enabling A20\r\n"
 msg_unreal: .asciz      " * Unreal Mode\r\n"
 msg_getmap: .asciz      " * Memory Map\r\n"
+msg_mkheap: .asciz      " * Making Space\r\n"
 msg_paging: .asciz      " * Paging\r\n"
 msg_fin:    .asciz      "Finished.\r\n"
 msg_aerr:   .asciz      "panic: Unable to allocate memory.\r\n"
 
+heap_start: .long       0
+heap_size:  .long       0
+
             // Long mode initial page tables
 pd_ptr:     .long       0
 pdp_ptr:    .long       0