diff --git a/lib/rex/assembly/nasm.rb b/lib/rex/assembly/nasm.rb
index e2ab338397..281ac48d1f 100644
--- a/lib/rex/assembly/nasm.rb
+++ b/lib/rex/assembly/nasm.rb
@@ -37,7 +37,7 @@ class Nasm
 	#
 	# Assembles the supplied assembly and returns the raw opcodes.
 	#
-	def self.assemble(assembly)
+	def self.assemble(assembly, bits=32)
 		check
 
 		# Open the temporary file
@@ -48,7 +48,7 @@ class Nasm
 		opath = tmp.path + '.out'
 
 		# Write the assembly data to a file
-		tmp.write("BITS 32\n" + assembly)
+		tmp.write("BITS #{bits}\n" + assembly)
 		tmp.flush()
 		tmp.seek(0)
 
@@ -70,7 +70,7 @@ class Nasm
 	#
 	# Disassembles the supplied raw opcodes
 	#
-	def self.disassemble(raw)
+	def self.disassemble(raw, bits=32)
 		check
 
 		tmp = Tempfile.new('nasmout')
@@ -82,7 +82,7 @@ class Nasm
 		tfd.flush()
 		tfd.close
 
-		p = ::IO.popen("\"#{@@ndisasm_path}\" -u \"#{tmp.path}\"")
+		p = ::IO.popen("\"#{@@ndisasm_path}\" -b #{bits} \"#{tmp.path}\"")
 		o = ''
 
 		begin
diff --git a/tools/nasm_shell.rb b/tools/nasm_shell.rb
index 262539a8c8..5a2859cd71 100755
--- a/tools/nasm_shell.rb
+++ b/tools/nasm_shell.rb
@@ -22,6 +22,12 @@ rescue RuntimeError
 	exit
 end
 
+bits = ARGV.length > 0 ? ARGV[0].to_i : 32
+if ! [16, 32, 64].include?(bits) then
+    puts "#{bits} bits not supported"
+    exit 1
+end
+
 # Start a pseudo shell and dispatch lines to be assembled and then
 # disassembled.
 shell = Rex::Ui::Text::PseudoShell.new("%bldnasm%clr")
@@ -35,7 +41,7 @@ shell.run { |line|
 
 	begin
 		puts(Rex::Assembly::Nasm.disassemble(
-			Rex::Assembly::Nasm.assemble(line)))
+			Rex::Assembly::Nasm.assemble(line, bits), bits))
 	rescue RuntimeError
 		puts "Error: #{$!}"
 	end