Add initial register states to meta section; remove entry directive

It makes sense to allow users to set the initial register values. This
allows someone to e.g. enable interrupts once the VM has started, or set
the initial stack pointer value.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/libvm/examples/deadbeef.asm

@@ -40,6 +40,13 @@
     .export ivt
 }
 
-.meta {
-    entry: main
+.section stack $0x4000 .. $0x5000 {
+    stack_base:
+    .export stack_base
+}
+
+.meta {
+    ip:     main
+    sp:     stack_base
+    ivt:    ivt
 }

src/libvm/examples/factorial.asm

@@ -24,5 +24,5 @@
 }
 
 .meta {
-    entry: main
+    ip: main
 }

src/libvm/examples/strings.asm

@@ -26,5 +26,5 @@
 }
 
 .meta {
-    entry: main
+    ip: main
 }

src/libvm/src/state.rs

@@ -1,4 +1,13 @@
-use crate::{addr::*, error::*, flags::*, inst::*, interrupt::*, mem::*, obj::obj::*, reg::*};
+use crate::{
+    addr::*,
+    error::*,
+    flags::*,
+    inst::*,
+    interrupt::*,
+    mem::*,
+    obj::{obj::*, syn::parser},
+    reg::*,
+};
 use std::mem;
 
 pub struct State {
@@ -37,8 +46,32 @@ impl State {
                     }
                 }
                 Section::Meta(MetaSection { entries }) => {
-                    if let Some(addr) = entries.get("entry") {
-                        self.set_reg_unchecked(IP, *addr);
+                    macro_rules! meta_addr {
+                        ($($name:expr, $reg:expr);* $(;)?) => {{
+                            $(
+                                if let Some(value) = entries.get($name) {
+                                    self.set_reg_unchecked($reg, *value);
+                                }
+                            )*
+                        }};
+                    }
+
+                    meta_addr!("ip", IP);
+                    meta_addr!("sp", SP);
+                    meta_addr!("flags", FLAGS);
+                    meta_addr!("status", STATUS);
+                    meta_addr!("ivt", IVT);
+
+                    if let Some(value) = entries.get("fp") {
+                        self.set_reg_unchecked(FP, *value);
+                    } else {
+                        self.set_reg_unchecked(FP, self.get_reg_unchecked(SP));
+                    }
+
+                    for (reg, value) in entries.iter() {
+                        if let Some(reg) = parser::parse_reg(reg) {
+                            self.set_reg_unchecked(reg, *value);
+                        }
                     }
                 }
             }