The instruction set of most (32 bit and above) RISC microprocessors includes arithmetic and logic instructions that operate on three different registers (so-called 3-address instructions). Two registers provide the source operands for the operation, while a third register is used to store the operation result. For example,
reg_31 = reg_0 "+" reg_1 reg_12 = reg_12 "AND" reg_31 reg_1 = reg_1 "XOR" reg_1 ...
Typically, about 32 registers are provided, so that 5 bits are sufficient to address one register, and 15 bits are required to specify three registers. This easily fits into a typical 32 bit instruction word.
Obviously, a memory component with two independent read-ports and one independent write-port is required to support 3-address operations. The applet includes the corresponding RTLIB RegBank component (class hades.models.rtlib.memory.RegBank). Unlike the standard asynchronous RAM component, write-operations are controlled by the clock signal. Open the memory editor (popup-menu, edit component) to edit the register file contents and to watch the read- and write-accesses.
To execute an arithmetic or logic operation, first select the ALU function and the three memory addresses via the corresponding IpinVector switches. Then enable the ALU output tri-state buffer, select write-enable (active low), and generate a clock pulse (rising edge) to write the ALU output data to the selected target register.
Run the applet | Run the editor (via Webstart)