added syscall and jalr to assembler, also fixed v1 value

assembler.rb

@@ -6,7 +6,7 @@ def r0() 0 end # zero
 def at() 1 end # assembler temporary
 
 # subroutine return values, may be changed by subroutines
-def v0() 2 end ; def v0() 3 end
+def v0() 2 end ; def v1() 3 end
 
 # subroutine arguments, may be changed by subroutines
 def a0() 4 end ; def a1() 5 end
@@ -193,6 +193,24 @@ class Assembler
 		word op
 	end
 
+	# TODO: actually encode "comment" argument
+	def syscall(comment)
+		#      iiiiii ccccc ccccc ccccc ccccc iiiiii
+		op = 0b000000_00000_00000_00000_00000_001100
+		word op
+	end
+
+	# TODO: expose the return address storage register
+	# NOTE: cannot use the same register for both operands
+	def jalr(reg)
+		# jumps to addr in j and stores return adr in r
+		#      iiiiii jjjjj ----- rrrrr ----- iiiiii
+		op = 0b000000_00000_00000_00000_00000_001001
+		op |= (reg & 0b11111) << 21
+		op |= (ra) << 11 # ra
+		word op
+	end
+
 	# # # PSEUDO-INSTRUCTIONS # # #
 	def nop
 		word 0x00000000