Dumb CPU

Dumb CPU, written in VHDL, is just meant for educational and experimental purposes. DCPU has a data bus width of 8 bits and address space width of 6 bits, thus addressing 64 bytes only. The lower 32 bytes of the address space are devoted to instructions whereas the upper 32 bytes are devoted to data. DCPU has an accumulator and a X register. There's no status register as all branching instructions are based on accumulator vs X register comparison.

Instruction decoding

Higher three bits (7,6,5) of each instructions are dedicated to instruction opcode. The remaining five bits contain the target address for instructions that deal with a target address (loads, stores, branches) and contain additional opcode space for instructions that do not require a target address.

Instructions with target address

High opcode	Mnemonic	Purpose
000	LDA	LoaD Accumulator
001	STA	STore Accumulator
010	LDX	LoaD X register
011	JMP	JuMP to address
100	BNE	Branch if accumulator is Not Equal to X register
101	BEQ	Branch if accumulator is EQual to X register
110	TBD
111	xxx	Other instructions decoding

Instructions without target address

Low opcode	Mnemonic	Purpose
00000	ZEA	Zero Accumulator
00001	INC	INCrement accumulator
00010	DTO	DaTa Out
.....	xxx	To be defined

TODO List

• Add and test Blinky program
• Organize project folders and constraints files
• Add projects for other FPGAs
• Add platform_top for integrating perpherals (SSD, UART, SPI), muxes, clock dividers and so on
• Add arithmetic instructions: ADD, SUB, AND, ORA, XOR
• Add NOP instructions, with/without argument (number of NOP cycles)
• Add instruction to emit pulse, useful to trigger external peripherals (STroBe)
• Add instruction to wait until the level of an input signal goes high. Usefult to wait peripheral completion
• Add instruction to select one out of two or four output selects. Useful for muxing external peripherals
• Add bootloader that loads program from SPI in another memory bank and jumps to it. Need specific instruction (BNK) to switch banks and
• Strikeme