SAP1 Breadboard Computer

The "Simple as Possible" design of computer.

Features

• 8-bit shared data/address bus.
• 256 bytes memory.
• Variable rate clock, between 1hz and 10khz, with manual step mode.
• Supports adding and subtracting!
• Display in signed or unsigned mode.

Future Work

• More ALU operations including OR, AND, XOR
• Conditional Branching
• Storage module

Example

add.asm

    mov a, #14
    add a, #28
    out a
    hlt

fibonacci.asm

    mov a, #0
    mov @99, a
    mov a, #1
    mov @100, a
loop:
    mov a, @99
    add a, @100
    mov @100, a
    mov @99, b
    out a
    jmp @loop

Instructions

Notation

[label:] <verb> [args...]

• A - Refers to the A-register.
• B - Refers to the B-register.
• #lit - A literal value given by the number lit, can be between 0 and 255.
• @loc - Refers to a location- literal address or value at a labeled location.
  • When loc is a number, this is considered an address in memory. Values between 0 and 255 are accepted as RAM is only 8-bit addressable.
  • When loc is an identifier, this is either the name of a register or the name of a label. When the name of a register is used, the location is the memory address given by the value stored in the register. When a label is given, the location is computed by the assembler then inserted in place of the label.

Utility

Instruction	Cycle Count	Description
nop	2-cylces	No-operation will be executed for a single.
hlt	3-cycles	Stop the cpu clock. No further operations will be run until reset button is pressed.

Output

out <source> - Sets the displayed value to be that of <source>.

Instruction	Cycle Count	Description
out @47	5-cylces	Output the value at literal memory address.
out #23	4-cylces	Output a literal value.
out A	3-cycles	Output the value in the A-register.
out B	3-cycles	Output the value in the B-register.

Move

mov <target>, <source> - Move a word value from one location to another. This instruction handles moving into and out of registers from memory, between registers, and setting literal values into the destination location.

Instruction	Cycle Count	Description
mov A, @47	4-cycles	Move the value at a literal memory address into the A-register.
mov A, #23	4-cylces	Move a literal value into the A-register.
mov A, B	3-cylces	Move the value in the B-register into the A-register.
mov B, @47	4-cycles	Move the value at a literal memory address into the B-register.
mov B, #23	4-cycles	Move a literal value into the B-register.
mov B, A	3-cycles	Move the value in the A-register into the B-register.
mov @47, A	5-cycles	Move the value in the A-register to the memory location given by a literal.
mov @47, B	5-cycles	Move the value in the B-register to the memory location given by a literal.
mov @47, #23	Unimplemented	Move a literal value into the memory location given by a literal.

Addition

add <target>, <source> - Perform addition such that A = A + <source>

Instruction	Cycle Count	Description
add A, @47	6-cycles	Add the value at a literal memory location to the value stored in the A-register, the result is stored back in the A-register.
add A, #23	5-cycles	Add a literal value to the value stored in the A-register, the result is stored back in the A-register.

Subtraction

sub <target>, <source> - Perform subtraction such that A = A - <source>.

Instruction	Cycle Count	Description
sub A, @47	6-cycles	Subtract the value at a literal memory location from the value stored in the A-register, the result is stored back in the A-register.
sub A, #23	5-cycles	Subtract a literal value from the value stored in the A-register, the result is stored back in the A-register.

Jump

jmp <target> - Unconditionally update the Program Counter so the next cycle will read the instruction at <target> location in memory.

Instruction	Cycle Count	Description
jmp #23	4-cycles	Set a literal memory location as the next location to be executed.
jmp @lbl	4-cycles	Set the next location to be executed to the memory location at a label.