ToyCPU-4bit/DESIGN.md

CPU Design

Memory segments

There are two separate memory segments:

• CODE segment is size of 256 bytes and is byte-addressed;
• DATA segment is size of 16 bytes and is byte-addressed.

To prevent confusion, we will append segment suffixes to differentiate between different addresses:

• 0x00_c — 8-bit address in CODE segment;
• 0x0_d — 4-bit address in DATA segment.

Registers

IP — Instruction Pointer, 8-bit register, points to next instruction in CODE segment. Cannot be directly accessed. Is automatically advanced after execution of every instruction.

R0 — First register of ALU. Always used as destination and as first operand for all ALU operations.

R1 — Second register of ALU. Always used as second operand for all ALU operations.

C — 1-bit carry flag.

ISA

LOAD/STORE

+—+—+—+—+———————+
|0|0|M|R|A A A A|
+—+—+—+—+———————+

M — 0 for load (read byte from memory and put in ALU register), 1 for store (read byte from ALU register and put in memory).

R — 0 to use R0 register, 1 to use R1 register.

AAAA — 4-bit address in DATA segment.

LOAD IMMEDIATE

+—+—+—+—+———————+
|0|1|H|R|I I I I|
+—+—+—+—+———————+

H — 0 to use 4 least significant bits, 1 to use 4 most significant bits.

R — 0 to use R0 register, 1 to use R1 register.

IIII — 4-bit immediate value.

NEAR JUMP

+—+—+—+—+———————+
|1|0|C|D|O O O O|
+—+—+—+—+———————+

C — 0 to ignore carry flag and jump unconditionally, 1 to only jump if carry flag is set.

D — direction of jump, 1 to jump forward, 0 to jump backward.

OOOO — jump offset.

FAR JUMP

+—+—+—+—+—+—+—+—+
|1|0|0|0|0|0|R|C|
+—+—+—+—+—+—+—+—+

Jumps at CODE segment address.

R — 0 to use address stored in R0 register, 1 to use address stored in R1 register.

C — 0 to ignore carry flag and jump unconditionally, 1 to only jump if carry flag is set.

COMPARISON

equal

+—+—+—+—+—+—+—+—+
|1|0|0|0|0|1|0|0|
+—+—+—+—+—+—+—+—+

Write 1 to C if value in R0 equals value in R1, write 0 otherwise.

greater

+—+—+—+—+—+—+—+—+
|1|0|0|0|0|1|0|1|
+—+—+—+—+—+—+—+—+

Write 1 to C if value in R0 is greater than value in R1, write 0 otherwise.

less

+—+—+—+—+—+—+—+—+
|1|0|0|0|0|1|1|0|
+—+—+—+—+—+—+—+—+

Write 1 to C if value in R0 is less than value in R1, write 0 otherwise.

not

+—+—+—+—+—+—+—+—+
|1|0|0|0|0|1|1|1|
+—+—+—+—+—+—+—+—+

Flips value in C. Use it to implement 'not equal', 'greater or equal', and 'less or equal' comparisons.

BIT SHIFT

+—+—+—+—+—+—————+
|1|0|1|R|M|L L L|
+—+—+—+—+—+—————+

R — 0 to shift bits in R0 register, 1 in R1 register.

M — mode of operation, 0 — logical shift, 1 — circular shift.

INCREMENT

+—+—+—+—+—+—+—+—+
|1|1|0|0|0|0|0|R|
+—+—+—+—+—+—+—+—+

Increments ALU register and sets C if result is zero.

R — 0 to increment R0 and 1 to increment R1.

DECREMENT

+—+—+—+—+—+—+—+—+
|1|1|0|0|0|0|1|R|
+—+—+—+—+—+—+—+—+

Decrements ALU register and sets C if result is zero.

R — 0 to decrement R0 and 1 to decrement R1.

ADD

+—+—+—+—+—+—+—+—+
|1|1|0|0|0|1|0|0|
+—+—+—+—+—+—+—+—+

Adds values from R0 and R1 and stores result in R0. Sets C if result overflows.

SUBTRACT

+—+—+—+—+—+—+—+—+
|1|1|0|0|0|1|0|1|
+—+—+—+—+—+—+—+—+

Subtracts R1 from R0 and stores result in R0. Sets C if result underflows.

AND

+—+—+—+—+—+—+—+—+
|1|1|0|0|0|1|1|0|
+—+—+—+—+—+—+—+—+

Bitwise AND operation using values from R0 and R1 and storing result in R0.

OR

+—+—+—+—+—+—+—+—+
|1|1|0|0|0|1|1|1|
+—+—+—+—+—+—+—+—+

Bitwise OR operation using values from R0 and R1 and storing result in R0.

XOR

+—+—+—+—+—+—+—+—+
|1|1|0|0|1|0|0|0|
+—+—+—+—+—+—+—+—+

Bitwise XOR operation using values from R0 and R1 and storing result in R0.

XNOR

+—+—+—+—+—+—+—+—+
|1|1|0|0|1|0|0|1|
+—+—+—+—+—+—+—+—+

Bitwise XNOR operation using values from R0 and R1 and storing result in R0.

ONE'S COMPLEMENT

+—+—+—+—+—+—+—+—+
|1|1|0|0|1|0|1|R|
+—+—+—+—+—+—+—+—+

Flip all bits in ALU register.

R — 0 to use R0, 1 to use R1.