Assignment 8: MINIAC

Due: 5:00pm, Friday, October 21. Value: 50 pts. Submit to Sauron.

Using Logisim, design a circuit implementing the MINIAC computer described below. You may use any built-in Logisim components in completing this assignment. The following table lists useful files for your work; to download them, you may need to right-click the link and select Save Link As….

circuit-miniac.circ:	Logisim starter circuit including clock, registers, ROM, and RAM.
miniac.py:	Python script for assembling and executing MINIAC assembly language files. Documentation is at top of the Python file.
miniac-halve.s:	MINIAC assembly language for halving a number.
miniac-mult.s:	MINIAC assembly language for multiplying two numbers.

The MINIAC instruction set was designed for the purpose of this assignment. It uses two one-byte registers, known as PC (program counter) and AC (accumulator). It includes a program memory containing 256 bytes implemented as read-only memory (ROM), and it has a data memory containing 32 bytes implemented as random-access memory (RAM).

Every MINIAC instruction is encoded in one byte: The top three bits give the operation to be performed, and the bottom five bits are an argument supporting the operation. Each of the three-bit combinations are tabulated below.

000xxxxx	BZ x	Branch if Zero
if AC = 0 then PC ← PC + signed(x) else PC ← PC + 1
001xxxxx	BN x	Branch if Negative
if AC < 0 then PC ← PC + signed(x) else PC ← PC + 1
010xxxxx	ST x	STore
RAM[x] ← AC; PC ← PC + 1
011xxxxx	RL x	Rotate Left
AC ← AC rotated left by distance given in lower 3 bits of x; PC ← PC + 1
100xxxxx	LI x	Load Immediate
AC ← signed(x); PC ← PC + 1
101xxxxx	LM x	Load from Memory
AC ← RAM[x]; PC ← PC + 1
110xxxxx	AD x	ADd
AC ← AC + RAM[x]; PC ← PC + 1
111xxxxx	SB x	SuBtract
AC ← AC − RAM[x]; PC ← PC + 1

Here is an example program fragment that takes a value n from RAM[1] and places n / 2 into RAM[0]. This is essentially the code found in the miniac-halve.s file.

hex	binary	assembly	comment
0xA1	101 00001	LM 1	# Load number to be halved,
0x67	011 00111	RL 7	# rotate it right once,
0x40	010 00000	ST 0	# and store result.
0x23	001 00011	BN 3	# Go to (b) if top bit is 1 (1's bit before rotation)
0x80	100 00000	LI 0	# Stop computer.
0x00	000 00000	BZ 0
0x81	100 00001	LI 1	# (b): Set AC to 0x80.
0x67	011 00111	RL 7
0xC0	110 00000	AD 0	# Add 0x80 to result to clear top bit.
0x40	010 00000	ST 0
0x80	100 00000	LI 0	# Stop computer.
0x00	000 00000	BZ 0

How to test your circuit

To test your circuit, you will want to load a program into the MINIAC's ROM memory. I suggest using the provided Python script for this. The script has two options: The -s option produces hexadecimal output corresponding to the machine code, and the -v option traces what happens as a MINIAC computer executes the machine code. The hexadecimal output of the -s option is in a format that Logisim accepts for its ROM circuits.

You'll want to save the Python script's hexadecimal output into a file. You can do this using output redirection.

linux% python miniac.py -s assem-filename.s > machine-filename.hex

Once you save into the file, go your circuit in Logisim, right-click (or control-click) the ROM, select Load Image…, and select the name of the file containing the hexadecimal code (machine-filename.hex in the above command). You will then want to start the clock ticking, either manually by clicking the clock component repeatedly or by selecting Enable Ticks from the Simulate menu.