kernel: allow for cases with only a small window in the base page

If you have a 4x16K or similar mapping model and most of the upper page
is taken up by discard and common then there may not be 4K to randomly play with.

In that case firstly we overflow and miscalculate the pages, secondly we then
scribble on stuff we shouldn't.

Instead tighten it up so there needs to be only 512 bytes (plus vectors etc
- 256 bytes on Z80) of space.

diff --git a/Kernel/start.c b/Kernel/start.c
index f54ee75..0f0d23f 100644 (file)
--- a/Kernel/start.c
+++ b/Kernel/start.c
@@ -83,7 +83,7 @@ void create_init(void)
 {
        uint8_t *j;
 
-       udata.u_top = PROGLOAD + 4096;  /* Plenty for the boot */
+       udata.u_top = PROGLOAD + 512;   /* Plenty for the boot */
        init_process = ptab_alloc();
        udata.u_ptab = init_process;
        init_process->p_top = udata.u_top;
@@ -99,8 +99,10 @@ void create_init(void)
                *j = NO_FILE;
        }
        /* Poke the execve arguments into user data space so _execve() can read them back */
+       /* Some systems only have a tiny window we can use at boot as most of
+          this space is loaded with common memory */
        argptr = PROGLOAD;
-       progptr = PROGLOAD + 2048;
+       progptr = PROGLOAD + 256;
 
        uzero((void *)progptr, 32);
        add_argument("/init");
@@ -112,7 +114,7 @@ void complete_init(void)
        uputp(0, (void *)argptr);
        /* Set up things to look like the process is calling _execve() */
        udata.u_argn2 = (arg_t)argptr; /* Environment (none) */
-       udata.u_argn =  (arg_t)PROGLOAD + 2048; /* "/init" */
+       udata.u_argn =  (arg_t)PROGLOAD + 256; /* "/init" */
        udata.u_argn1 = (arg_t)PROGLOAD; /* Arguments */
 
 #ifdef CONFIG_LEVEL_2