README.md

uf16

The UF16 is a 16-bit stack machine with several notable features:

• Compact area footprint
• Low-latency interrupt response
• Support for preemptive multitasking
• Low power consumption with a sleep instruction
• Compatibility with the Forth programming language

The Forth language offers an interactive interface to hardware, simplifying the software/hardware debugging process. Additionally, it generates highly compact code, reducing memory size requirements. To enhance Forth performance, the UF16 includes a fast call instruction and hardware tail call optimization.

The UF16 employs a three-stage pipeline (fetch, decode, and execute) that maximizes memory performance, with a clock period equal to memory access time. Moreover, the short pipeline results in a low branch penalty, which is particularly beneficial since Forth relies heavily on branch and call instructions compared to other programming languages.

Architecture

The UF16 architecture was specifically designed with a compact area footprint in mind, followed by enhancements aimed at improving speed performance. It features two stacks: a data stack and a return stack, both of which reside in memory. This design choice minimizes gate count and facilitates rapid task switching.

Utilizing a Von Neumann architecture, the UF16 accesses code, data, and I/O through a single bus, which is also employed for push and pop instructions. Additionally, some internal registers are utilized to cache push and pop data, effectively reducing memory bandwidth pressure.

In the UF16 architecture, the DSP (Data Stack Pointer) and RSP (Return Stack Pointer) internal registers are designated for managing the data and return stacks, respectively. Push and pop instructions automatically update the DSP and RSP registers. The DSPB and RSPB registers indicate the bottoms of the data and return stacks and are used to check for underflow exceptions.

The T register holds the top of the data stack (TOS), while the N register contains the next item below the TOS. The R register maintains the top of the return stack. The push instruction writes a new item to RAM and updates the DSP or RSP only when the cache queue is full. Conversely, the pop instruction reads an item from RAM and updates the DSP or RSP only when the cache queue is empty.

When the data stack is empty, the condition is indicated by DSP=DSPB, and for the return stack, it is RSP=RSPB. The data stack begins at DSPB+1 and grows towards higher memory addresses, while the return stack starts at RSPB−1 and grows towards lower memory addresses.

A stack overflow is signaled when the push instruction causes the DSP to equal the RSP. Both stack underflow and overflow events trigger a hardware exception, leading to a jump to the service routine specified by the EXCEPTION_ADDR parameter. Hardware interrupts are indicated by the irq_i input pin, causing a jump to the service routine defined by the irqaddr_i input pins.

In the UF16 architecture, a single instruction can perform two memory accesses: one for data reading and another for data writing. The memory read operation is initiated at the end of the decode stage, with the read data being stored in registers by the end of the execute stage. In contrast, the memory write operation is launched at the end of the execute stage.

The PC (Program Counter) register points to the address of the next instruction and is typically incremented by 1. A code jump is executed by loading the PC with the instruction register (IR) when the jump address is specified as immediate data. Alternatively, it can be loaded with the R register, for example, when returning from a subroutine.

Data loading between registers or between registers and memory is selectable within the instruction code. This flexibility allows for the specification of how to load each register within the same instruction.

Instruction Set

In the UF16 architecture, the instruction size is consistently two bytes for all instructions, except for those that require an immediate operand, such as branching to an address or loading immediate data. For these instructions, an additional two-byte operand is necessary.

However, there are equivalent two-byte instructions that utilize reduced-size operands. For example, the call instruction has a 2 + 2 byte version, where the last two bytes specify the callee address. There is also a compact version that consists of only 2 bytes, where the most significant bit (MSB) is set to 1 to distinguish it from other instructions. The lower 15 bits of this instruction represent the callee address, which is sufficient to address the entire addressable space.

	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
ucode	0	0	u₁₃	u₁₂	u₁₁	u₁₀	u₉	u₈	u₇	u₆	u₅	u₄	u₃	u₂	u₁	u₀
branch	0	1	0	0	r₁	r₀	a₉	a₈	a₇	a₆	a₅	a₄	a₃	a₂	a₁	a₀
branchz	0	1	0	1	r₁	r₀	a₉	a₈	a₇	a₆	a₅	a₄	a₃	a₂	a₁	a₀
next	0	1	1	0	r₁	r₀	a₉	a₈	a₇	a₆	a₅	a₄	a₃	a₂	a₁	a₀
ldi	0	1	1	1	i₁₁	i₁₀	i₉	i₈	i₇	i₆	i₅	i₄	i₃	i₂	i₁	i₀
call	1	a₁₄	a₁₃	a₁₂	a₁₁	a₁₀	a₉	a₈	a₇	a₆	a₅	a₄	a₃	a₂	a₁	a₀

In the UF16 architecture, instructions fetched from memory are decoded during the decode stage, and the results are stored in the internal instruction register. This register consists of the UIR, which holds the instruction to be executed, and the LIT register, which contains any immediate data if present.

The UIR is composed of 18 bits, structured as follows:

• Bits 17-16: EXT_ALU[1:0]
  • These two bits select the meaning of the less significant 4 bits that define the ALU operation.
• Bits 15-14: EXT_RAM[1:0]
  • These two bits define the second RAM access operation.
• Bit 13: N[0]
  • This bit defines the load operation of the N register.
• Bits 12-11: T[1:0]
  • These two bits define the load operation of the T register.
• Bits 10-9: R[1:0]
  • These two bits define the load operation of the R register.
• Bits 8-7: PC[1:0]
  • These two bits define the load operation of the PC register.
• Bits 6-4: RAM[2:0]
  • These three bits define the RAM access operation.
• 3-0: ALU[3:0]
  • These four bits, in conjunction with EXT_ALU[1:0], define the ALU operation.

ucode Instruction

ucode instruction has the two MSB bits equal to '00'. The other bits define the operations that has to be executed according to following table:

#	N[0]=u₁₃	T[1:0]=u₁₂,u₁₁	R[1:0]=u₁₀,u₉	PC[1:0]=u₈,u₇	RAM[2:0]=u₆,u₅,u₄	ALU[3:0]=u₃,u₂,u₁,u₀
0	NOP	NOP	NOP	PC←PC+1	NOP	NOP
1	N←T	T←N	R←M1	PC←R	N←POP	NOT
2		T←R	R←PC	PC←IR	R←POP	AND
3		T←IR	R←T	PC←?IR	N←RAM	OR
4					T←RAM	XOR
5					PUSH←N	SHL
6					PUSH←R	SHR
7					RAM←N	ADD
8						SUB
9						MUL
10						DIV
11						ADD1
12						ADD2
13
14
15						CARRY

• N[0] bit defines if register N has to be loaded with the content of register T.
• T[1:0] bits define if register T has to be loaded with the content of one of the registers N, R or IR (immediate value).
• R[1:0] bits define if register R has to be loaded with the content of register T, or register PC or decremented by 1 (M1).
• PC[1:0] bits define how the program counter PC has to be loaded.
• RAM[2:0] bits define the memory access operation:
  • N←POP: pop data from data stack. This could require a memory read if data cache is empty.
  • PUSH←N: push data into data stack. This could require a memory write if data cache is full.
  • R←POP: pop data from return stack. This could require a memory read if return cache is empty.
  • PUSH←R: push data into return stack. This could require a memory write if return cache is full.
  • N←RAM: data read from memory into register N.
  • T←RAM: data read from memory into register T.
  • RAM←N: data write into memory from register N.
• ALU[3:0] bits define the ALU operation to be executed:
  • NOT: bit inversion of register T content.

#	EXT_ALU[1:0]	EXT_RAM[1:0]	N[0]	T[1:0]	R[1:0]	PC[1:0]	RAM[2:0]	ALU0[3:0]	ALU1[3:0]	ALU2[3:0]	ALU3[3:0]
0	ALU0	00	0	00	00	00	000	0000
1	ALU1	PUSH←N	N←T	T←N	R←M1	PC←R	N←POP	NOT
2	ALU2	PUSH←R		T←R	R←PC	PC←IR	R←POP	AND
3	ALU3	RAM←N		T←IR	R←T	PC←?IR	N←RAM	OR
4							T←RAM	XOR
5							PUSH←N	SHL	SHLC
6							PUSH←R	SHR	SHRC
7							RAM←N	ADD	ADDC
8								SUB	SUBC
9								MUL
10								DIV
11								ADD1	ADD1C
12								ADD2	ADD2C
13
14									IRQEN	IRQSET
15								CARRY	SLEEP	IRQDIS