MIC-1

The MIC-1 is a processor architecture invented by Andrew S. Tanenbaum to use as a simple but complete example in his teaching book Structured Computer Organization.

It consists of a very simple control unit that runs microcode from a 512-words store.

The Micro-Assembly Language (MAL) is engineered to allow simple writing of an IJVM interpreter, and the source code for such an interpreter can be found in the book.

Data path
The data path is the core of the MIC-1. It contains 32-bit registers, buses, an ALU and a shifter.

Buses
There are 2 main buses of 32 lines (or 32 bits) each:


 * B bus: connected to the output of the registers and to the input of the ALU.
 * C bus: connected to the output of the shifter and to the input of the registers.

Registers
Registers are selected by 2 control lines: one to enable the B bus and the other to enable the C bus. The B bus can be enabled by just one register at a time, since the transfer of data from 2 registers at the same time, would make this data inconsistent. In contrast, the C bus can be enabled by more than 1 register at the same time; as a matter of fact, the current value present in the C bus can be written to more than 1 register without problems.

The reading and writing operations are carried out in 1 clock cycle.

The MBR register is a readonly register, and it contains 2 control lines. Since it is an 8-bit register, its output is connected to the least significant 8 bits of the B bus. It can be set to provide its output in 2 ways:


 * 2's complement (MBR): all the remaining 24 bits of the B bus are set to 1, if it's a negative number, or they are set to 0, if it's a positive number (sign extension).
 * Without complement (MBRU): the remaining 24 bits (of 32 total) are set to 0.

ALU
The ALU (or arithmetic logic unit) has the following input, output and control lines:


 * 2 32-bit input lines: one for the B bus and one for the bus that is connected directly to the H register.
 * 1 32-bit output line, which is connected directly to the shifter.
 * 6 control lines aimed to select which operation to perform.
 * 2 other output lines for the status flags N (negative) and Z (zero).

Shifter
The shifter contains a 32-bit input and output. The output is connected directly to the C bus. The shifter is used to perform logical and arithmetic shift operations, by simply setting respectively the control signal SLL8 (Shift Left Logical) and SRA1 (Shift Right Arithmetic).