Register Reference vs Memory Reference Instructions (Mano Basic Computer - 16-bit)

In Mano's Basic Computer, instructions are 16-bit: I (15) | Opcode (14-12) | Address (11-0). Whether an instruction is Memory-reference, Register-reference or I/O is decided by the opcode and I bit.

1. Memory Reference Instructions ( $I \times 2^{12} + opcode$ )

Opcode 000 to 110 with $I=0/1$ = 7 instructions. Bits 11-0 give memory address; $I=0$ = direct, $I=1$ = indirect (EA = M[address]).

Opcode	Symbol	Operation	Example
000	AND	$AC \leftarrow AC \land M[EA]$	AND 100 / AND I 100
001	ADD	$AC \leftarrow AC + M[EA]$	ADD 200
010	LDA	$AC \leftarrow M[EA]$	LDA 300
011	STA	$M[EA] \leftarrow AC$	STA 400
100	BUN	$PC \leftarrow EA$	BUN 500 Unconditional branch
101	BSA	$M[EA]\leftarrow PC,\; PC\leftarrow EA+1$	BSA 600 Branch & save return
110	ISZ	$M[EA]\leftarrow M[EA]+1$, if $M[EA]=0$ then $PC\leftarrow PC+1$ (skip)	ISZ 700

Format: 1xxx xxxx xxxx xxxx where xxx=opcode.
E.g. LDA 0x005 direct: 0 010 000000000101 = 0x2005; indirect: 1 010 000000000101 = 0xA005.

Executed in Fetch -> Decode -> Execute using $T_0-T_4$. Need memory access in execute phase.

2. Register Reference Instructions

Recognized when opcode = 111 and I=0 (IR = 0111 0xxx xxxx xxxx). Bits 11-0 specify which micro-operation on AC, E, etc. No memory access, executed in one cycle ($T_3$). Bit =1 means operation performed; multiple bits can be 1 together.

Bit	Symbol	Micro-operation
11	CLA 7800	$AC \leftarrow 0$
10	CLE 7400	$E \leftarrow 0$
9	CMA 7200	$AC \leftarrow \overline{AC}$
8	CME 7100	$E \leftarrow \bar{E}$
7	CIR 7080	$AC,E \leftarrow shr(AC,E)$
6	CIL 7040	$AC,E \leftarrow shl(AC,E)$
5	INC 7020	$AC \leftarrow AC+1$
4	SPA 7010	If $AC(15)=0$ then $PC\leftarrow PC+1$ (skip if positive)
3	SNA 7008	Skip if $AC$ negative
2	SZA 7004	Skip if $AC=0$
1	SZE 7002	Skip if $E=0$
0	HLT 7001	$S\leftarrow 0$ halt

E.g. CLA = 0111 1000 0000 0000 = 0x7800; CMA INC (complement and increment = negate) = 0x7200|0x7020 = 0x7220.

Key difference:

Feature	Memory-Reference	Register-Reference
Opcode	000-110	111
I bit	selects direct/indirect	must be 0
Operands	in Memory	in AC/E/PC
Cycles	needs $M[EA]$ access	single micro-op, no memory
Example	ADD 0100 does $AC+M[0100]$	CLA clears AC

I/O instructions are third type: opcode 111, I=1, bits 11-0 select I/O micro-op e.g. INP, OUT.