diff --git a/assets/bios/mon.js b/assets/bios/mon.js
new file mode 100644
index 0000000..505ba1c
--- /dev/null
+++ b/assets/bios/mon.js
@@ -0,0 +1,101 @@
+/*
+C O M M A N D S
+
+L (srec literal) : load the s-record into the memory address specified by the s-record. Pointer will also be
+                    moved to the specified start address if such field exists; otherwise untouched
+J (hex addr) : jump to the program stored in the address
+R : execute the code starting from the current pointer
+M (hex addr) : shows a byte on the address
+N : Increment the memory pointer, then show the byte on the address
+W (hex string) : write given byte string to the memory starting from the current pointer. If the length of the
+                    hex string is odd, error will be thrown. The pointer will auto-increment.
+
+Prompt on successful: .
+Prompt on error: ?
+
+ */
+
+
+let uhex = (i, len) => { return (i >>> 0).toString(16).toUpperCase().padStart(len||2, '0').slice(-(len||2)) }
+
+println(`/MONITOR/  ${sys.maxmem()} BYTES SYSTEM`)
+
+let prompt = ['?','.']
+
+let mode = "M"
+let ptr = 0
+let previousError = undefined
+
+while (1) {
+    print(prompt[+!previousError])
+    let buf = read().split(' ')
+    let cmd = buf[0].toUpperCase()
+
+    if ("M" == cmd) {
+        let addr = parseInt(buf[1], 16)
+        let addr2 = parseInt(buf[2], 16)
+        if (Number.isNaN(addr)) {
+            println((uhex(ptr, 4))+' : '+uhex(sys.peek(ptr)))
+            previousError = undefined
+        }
+        else {
+            let oldptr = ptr
+            ptr = addr
+            if (Number.isNaN(addr2)) {
+                println(uhex(sys.peek(ptr)))
+                previousError = undefined
+            }
+            else if (Math.abs(addr2) <= Math.abs(addr))
+                previousError = "Range error: end is greater than start"
+            else {
+                for (let i = 0; i <= Math.abs(addr2) - Math.abs(addr); i++) {
+                    if (i % 16 == 0 && i > 0) { println() }
+                    if (i % 16 == 0) { print((uhex(ptr, 4))+' : ') }
+                    print(uhex(sys.peek(ptr)) + ' ')
+                    if (addr < 0 && addr2 < 0) { ptr-- } else { ptr++ }
+                }
+                println()
+                previousError = undefined
+            }
+            ptr = oldptr
+        }
+    }
+    else if ("N" == cmd) {
+        ptr++
+        println((uhex(ptr, 4))+' : '+uhex(sys.peek(ptr)))
+        previousError = undefined
+    }
+    else if ("J" == cmd) {
+        let addr = parseInt(buf[1], 16)
+        if (Number.isNaN(addr))
+            previousError = "Jump address unspecified"
+        else {
+            ptr = addr
+            previousError = undefined
+        }
+    }
+    else if ("W" == cmd) {
+        let arg = buf[1]
+        if (arg == undefined)
+            previousError = "No arguments given"
+        else if (arg.length % 2 == 1)
+            previousError = "Length of byte string is odd number"
+        else {
+            for (let i = 0; i < arg.length; i += 2) {
+                let b = parseInt(arg.charAt(i)+arg.charAt(i+1), 16)
+                sys.poke(ptr++, b)
+            }
+            previousError = undefined
+        }
+    }
+    else if ('' == cmd) {
+        // do nothing
+    }
+    else if ('?' == cmd) {
+        println(previousError)
+    }
+    else {
+        previousError = "Unknown command"
+    }
+
+}