Fix jalv offset -- negative values allowed

basic/src/bin/valida.rs

@@ -147,7 +147,7 @@ fn show_frame(args: ArgMatches, context: &mut Context) -> Result<Option<String>>
         let offset = i * -4;
         let read_addr = (fp + offset) as u32;
         let string_val = context.machine_.mem().examine(read_addr);
-        let frameslot_addr = format!("{}(fp)", offset);
+        let frameslot_addr = format!("0x{:8} | {:3}(fp)", read_addr, offset);
         let frameslot = format!("{:>7}", frameslot_addr);
         let frame_str = format!("\n{} : {}", frameslot, string_val);
         frame += &frame_str;

cpu/src/lib.rs

@@ -521,7 +521,8 @@ where
             .mem_mut()
             .read(clk, read_addr, true, pc, opcode, 2, "")
             .into();
-        state.cpu_mut().fp += cell;
+        let offset: i32 = cell as i32;
+        state.cpu_mut().fp = (state.cpu().fp as i32 + offset) as u32;
         state.cpu_mut().push_op(Operation::Jalv, opcode, ops);
     }
 }