diff --git a/src/main.rs b/src/main.rs
index d306bf2..52e84ac 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -46,6 +46,7 @@ struct EmuState {
     cpu: M68K,
     symbol_tables: SymbolTables,
     address_breakpoints: IndexSet<u32>,
+    last_peek_format: (peek::Format, peek::Size),
 }
 
 /// 68K Backplane Computer Emulator
@@ -75,6 +76,7 @@ fn main() -> Result<(), anyhow::Error> {
         cpu: M68K::new(backplane),
         symbol_tables,
         address_breakpoints: IndexSet::new(),
+        last_peek_format: (peek::Format::Hex, peek::Size::Byte),
     };
     if args.run {
         let mut out = String::new();
@@ -241,23 +243,30 @@ fn main() -> Result<(), anyhow::Error> {
             .arg(
                 Arg::new("fmt")
                     .short('f')
-                    .required(true)
                     .takes_value(true)
                     .help("The format to print the values in (<fmt><size>)"),
             )
             .arg(Arg::new("addr").required(true))
             .about("Peek a memory address"),
         |args, state| {
-            let fmt_str = args.get_one::<String>("fmt").unwrap();
-            if fmt_str.len() != 2 {
-                return Err(anyhow!("Peek format length must be 2"));
-            }
-            let fmt = peek::Format::try_from(fmt_str.chars().next().unwrap())?;
-            let size = peek::Size::try_from(fmt_str.chars().nth(1).unwrap())?;
+            let (fmt, size) = if let Some(fmt_str) = args.get_one::<String>("fmt") {
+                if fmt_str.len() != 2 {
+                    return Err(anyhow!("Peek format length must be 2"));
+                }
+                let fmt = peek::Format::try_from(fmt_str.chars().next().unwrap())?;
+                let size = peek::Size::try_from(fmt_str.chars().nth(1).unwrap())?;
+                state.last_peek_format = (fmt, size);
+                (fmt, size)
+            } else {
+                state.last_peek_format
+            };
             let count = parse::<u32>(args.get_one::<String>("count").map_or("1", String::as_str))?;
             let addr = state
                 .symbol_tables
                 .parse_location_address(args.get_one::<String>("addr").unwrap())?;
+            if (size != peek::Size::Byte) & ((addr & 0x1) != 0) {
+                    return Err(anyhow!("Cannot peek words from a non-aligned address"));
+            }
             let mut data = Vec::new();
             let bus = state.cpu.bus_mut();
             for i in 0..count {
diff --git a/src/peek.rs b/src/peek.rs
index b132ff5..157268f 100644
--- a/src/peek.rs
+++ b/src/peek.rs
@@ -1,6 +1,6 @@
 use thiserror::Error;
 
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum Format {
     Octal,
     Hex,
@@ -47,7 +47,7 @@ impl TryFrom<char> for Format {
     }
 }
 
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum Size {
     Byte,
     Word,