diff --git a/host/assembler.fnl b/host/assembler.fnl
new file mode 100644
index 0000000..ae638d8
--- /dev/null
+++ b/host/assembler.fnl
@@ -0,0 +1 @@
+(local opcodes (require :host.assembler.opcodes))
diff --git a/host/assembler/elf.fnl b/host/assembler/elf.fnl
index 18b8c32..67f86a7 100644
--- a/host/assembler/elf.fnl
+++ b/host/assembler/elf.fnl
@@ -1,5 +1,3 @@
-(local {: print-string} (require :print))
-
 (local byte string.char)
 (fn word->byte [x]
 	"convert a 32 bit word into a little endian byte string"
@@ -70,7 +68,3 @@
 					:section-header-count 0
 					:string-section-index 0
 					:phead-count 0}))
-
-(with-open [f (io.open :test.elf :w)]
-	(f:write elf))
-(print-string elf)
diff --git a/host/assembler/opcodes.fnl b/host/assembler/opcodes.fnl
new file mode 100644
index 0000000..25403ce
--- /dev/null
+++ b/host/assembler/opcodes.fnl
@@ -0,0 +1,92 @@
+(local util (require :host.util))
+
+(local conditions {:equal 0x0
+			 						 :not-equal 0x1
+									 :carry 0x2
+									 :not-carry 0x3
+									 :negative 0x4
+									 :positive 0x5
+									 :overflow 0x6
+									 :no-overflow 0x7
+									 :higher-unsigned 0x8
+									 :lower-unsigned 0x9
+									 :greater-equal-signed 0xa
+									 :less-signed 0xb
+									 :greater-signed 0xc
+									 :less-equal-signed 0xd
+									 :always 0xe})
+
+(fn r [n]
+	"Create a register for use in instructions"
+	{:register (case n
+							 :pc 0xf
+							 :lr 0xe
+							 :sp 0xd
+							 n n)})
+
+(fn register? [x]
+	(and (= (type x) :table) (. x :register)))
+
+
+(fn bit [flag]
+	"Convert a boolean to an integer for shifting"
+	(if flag 1 0))
+
+(fn calculate-rotation [x]
+	"Construct a valid arm rotation of x"
+	(faccumulate [acc nil i 0 32 2 &until acc]
+		(let [rotated (util.rotate-left x i)] (if (<= rotated 0xff) i))))
+
+(fn parse-conditions [?options]
+	(let [{:cond cond :flags set-flags} (or ?options {})
+				 set-flags (bit (or set-flags false))
+				 cond (. conditions (or cond :always))]
+		{: cond : set-flags}))
+
+(fn move [dest source ?options]
+	{:fnl/docstring "Set register *dest* to a value\n*source* can be a register or an immediate value smaller than 4096"
+	 :fnl/arglist [dest source & {: cond : flags}]}
+	(let [{: cond : set-flags} (parse-conditions ?options)
+				 source-register (register? source)
+				 dest (register? dest)]
+	 (when (not dest) (error "dest must be a register"))
+	 (if source-register
+				(bor 
+					(lshift cond 28)
+					(lshift 0xd 21)
+					(lshift set-flags 20)
+					(lshift dest 12)
+					source-register)
+				(let [rotation (calculate-rotation source)]
+					(when (= nil rotation) (error "Unencodable immediate value"))
+					(bor 
+						(lshift cond 28)
+						(lshift 1 25)
+						(lshift 0xd 21)
+						(lshift set-flags 20)
+						(lshift dest 12)
+						(lshift (/ rotation 2) 8)
+						(band (util.rotate-left source rotation) 0xff))))))
+
+(fn branch [offset ?options]
+	(let [{: cond} (parse-conditions ?options)
+				max-immediate 0xffffff
+			  offset (band offset util.word-max)]
+		(bor
+			(lshift cond 28)
+			(lshift 10 24)
+			(band (rshift offset 2) 0xffffff))))
+
+(fn branch-link [offset ?options]
+	(let [{: cond} (parse-conditions ?options)
+				max-immediate 0xffffff
+			  offset (band offset util.word-max)]
+		(bor
+			(lshift cond 28)
+			(lshift 11 24)
+			(band (rshift offset 2) 0xffffff))))
+
+{: r
+ : move
+ : branch
+ : branch-link}
diff --git a/host/assembler/print.fnl b/host/assembler/print.fnl
index 93c9a29..6cd24fc 100644
--- a/host/assembler/print.fnl
+++ b/host/assembler/print.fnl
@@ -1,42 +1,44 @@
-(local {:map map} (require :lume))
+(local {:map map} (require :deps.lume))
 
-(local string->bytes (fn [s]
-											 (table.pack (string.byte s 1 -1))))
+(fn string->bytes [s]
+	(table.pack (string.byte s 1 -1)))
 
-(local bytes->string (fn [x] (string.char (table.unpack x))))
+(fn bytes->string [x] (string.char (table.unpack x)))
 
-(local show-bytes (fn [byte-string]
-										"print a string as human readble bytes"
-										(let [formatted (map (string->bytes byte-string)
-																				 (λ [byte]
-																						(if byte
-																								(string.format :%02x byte)
-																								"##")))]
-											(table.concat formatted " "))))
+(fn show-bytes [byte-string]
+	"print a string as human readble bytes"
+	(let [formatted (map (string->bytes byte-string)
+											 (λ [byte]
+													(if byte
+															(string.format :%02x byte)
+															"##")))]
+		(table.concat formatted " ")))
 
-(local show-ascii
-			 (fn [byte-string] (-> byte-string
-									 (string->bytes)
-									 (map (lambda [byte] (if (< 31 byte 126) byte 46)))
-									 (bytes->string))))
+(fn show-ascii [byte-string] 
+	(-> byte-string
+			(string->bytes)
+			(map (lambda [byte] (if (< 31 byte 126) byte 46)))
+			(bytes->string)))
 
-(local pad-string (fn [s count]
-			 (string.format (.. :%- count :s) s)))
+(fn pad-string [s count]
+	(string.format (.. :%- count :s) s))
 
-(local print-string
-			 (fn [byte-string]
-					(let [row-len 8
+(fn print-string [byte-string]
+	(let [row-len 8
 								len (length byte-string)
 								full-rows (math.floor (/ len row-len))
 								rest (math.modf len full-rows)]
-						(for [i 0 full-rows 1]
-							(let [row (byte-string:sub (+ 1 (* row-len i)) (+ row-len (* row-len i)))]
-								(when (< 0 (length row)) (print (..
-												 :| (pad-string (show-bytes row) (- (* 3 row-len) 1))
-												 :|
-												 "   "
-												 :|
-												 (pad-string (show-ascii row) row-len)
-												 :|))))))))
+		(for [i 0 full-rows 1]
+			(let [row (byte-string:sub
+									(+ 1 (* row-len i))
+									(+ row-len (* row-len i)))]
+				(when (< 0 (length row)) 
+					(print (..
+									 :| (pad-string (show-bytes row) (- (* 3 row-len) 1))
+									 :|
+									 "   "
+									 :|
+									 (pad-string (show-ascii row) row-len)
+									 :|)))))))
 
 {: print-string}
diff --git a/host/assembler/util.fnl b/host/assembler/util.fnl
new file mode 100644
index 0000000..b078720
--- /dev/null
+++ b/host/assembler/util.fnl
@@ -0,0 +1,17 @@
+(local byte string.char)
+(fn word->byte [x]
+	"convert a 32 bit word into a little endian byte string"
+	(string.char
+		(-> x (band 0xff))
+		(-> x (rshift 8) (band 0xff))
+		(-> x (rshift 16) (band 0xff))
+		(-> x (rshift 24))))
+(fn half->byte [x]
+	"convert a 16 bit half-word into a little endian byte string"
+	(string.char
+		(-> x (band 0xff))
+		(-> x (rshift 8) (band 0xff))))
+
+{: byte
+ : word->byte
+ : half->byte}
diff --git a/host/util.fnl b/host/util.fnl
new file mode 100644
index 0000000..1d26ce8
--- /dev/null
+++ b/host/util.fnl
@@ -0,0 +1,34 @@
+(local byte string.char)
+(fn word->byte [x]
+	"convert a 32 bit word into a little endian byte string"
+	(string.char
+		(-> x (band 0xff))
+		(-> x (rshift 8) (band 0xff))
+		(-> x (rshift 16) (band 0xff))
+		(-> x (rshift 24))))
+(fn half->byte [x]
+	"convert a 16 bit half-word into a little endian byte string"
+	(string.char
+		(-> x (band 0xff))
+		(-> x (rshift 8) (band 0xff))))
+
+(local word-max 0xffffffff)
+
+(fn rotate-right [x n]
+	"Rotate *x* right as a 32 bit integer by *n* bits."
+	(bor (rshift x n) (band word-max (lshift x (- 32 n)))))
+(fn rotate-left [x n]
+	"Rotate *x* right as a 32 bit integer by *n* bits."
+	(bor (band word-max (lshift x n)) (rshift x (- 32 n))))
+
+(fn hex [x ?padding]
+	"Convert x to hexadecimal, optionally padding to ?padding characters"
+	(string.format (.. :% :0 (or ?padding 0) :x) x))
+
+{: byte
+ : word->byte
+ : half->byte
+ : rotate-left
+ : rotate-right
+ : hex
+ : word-max}