MythSim Microcode Language Quick Reference
Register Set
a_sel=n		Place the value in register n (0-7) on the a_bus.
b_sel=n		Place the value in register n (0-7) on the b_bus.
ri_sel		Store result in the register defined in ir_ri.
rj_sel		Place the value in the register defined by ir_rj on the a_bus.
rk_sel		Place the value in the register defined by ir_rk on the b_bus.
rn_write		Store the result_bus value into register n (0-7).
Arithmetic Logic Unit
alu_sel=	NOT	NOT(a_bus) => alu_bus
	OR	OR(a_bus, b_bus) => alu_bus
	AND	AND(a_bus, b_bus) => alu_bus
	XOR	XOR(a_bus, b_bus) => alu_bus
	ADD	a_bus + b_bus + c_in => alu_bus
	SUB	a_bus + NOT(b_bus) + c_in => alu_bus
	ADDA	a_bus + c_in => alu_bus
	SUBA	a_bus -1 + c_in => alu_bus
c_in		Set the c_in to 1 for use by ALU operations (4-7).
Memory Interface
result_sel=	ALU	Place the alu_bus value on the result_bus.
	MDR	Place the mdr value on the result_bus.
	IR_CONST4	Place ir_const4 on the result_bus. (ir_const4 = last 4 bits of ir0)
	IR_CONST8	Place ir_const8 on the result_bus. (ir_const8 = all 8 bits of ir0)
ir0_sel=	LOAD	Load the value on the memory_bus into ir0 (Instruction Register 0).
ir1_sel=	LOAD	Load the value on the memory_bus into ir1 (Instruction Register 1).
mar_sel=	LOAD	Load the value on the alu_bus into the mar (Memory Address Register).
mdr_sel=	LOAD_ALU	Load the value on the alu_bus into the mdr (Memory Address Register).
	LOAD_MEM	Load the value on the memory_bus into the mdr (Memory Data Register).
read		Place the value at the main memory address mar on the memory_bus.
write		Write the value of mdr to the main memory location mar.
Control Structures
goto label		Jump to the given label.
if status_line then goto label_1 (else goto label_2) endif		If the value of status_line=1 then goto label_1 else goto label_2.
		status_line	Description
		m_7	most significant bit from alu_bus
		c_out	carry bit from the alu_bus
		v	overflow bit from the ALU
		wait	wait signal from Main Memory

Control Unit Diagram

Datapath Diagram