fox32/docs/cpu.md

Fox32 CPU

This document aims to describe the CPU in the the Fox32 architecture. Peripherals such as the disk controller are described elsewhere.

Endianness

All 16-bit or 32-bit values are stored in memory in little-endian order.

Registers

The Fox32 CPU has the following registers:

• r0-r31: 32-bit general-purpose registers
• rsp: current stack pointer
• resp: exception stack pointer
• rfp: frame pointer
• rip: instruction pointer
• condition flags, which are updated after some operations
  • zero flag
  • carry flag
• other flags
  • enable use of exception stack pointer (swap sp)
  • interrupt flag, to enable interrupt handling
• MMU state
  • MMU enabled flag
  • page directory pointer

External buses

There are two kinds of external bus that the Fox32 CPU can address:

• Memory: Data is read from and written to memory with the mov instruction, instructions are fetch from memory.
• I/O bus: Peripherals are connected to the I/O bus. Peripheral registers can be read using the in instruction and written using the out instruction.

Instruction encoding

All instructions start with a 16-bit control word, which is optionally followed by a source operand, or by source and target operands, depending on the opcode.

bits	name	description
1:0	source	source operand type
3:2	target	target operand type
6:4	cond	condition code
7	—	reserved, must be zero
13:8	opcode	operation code/type, e.g. mov or add
15:14	size	operation size, e.g. 32 bits

Operand types

value	description	size of operand	what’s actually stored?
0	register	8 bits	register number
1	register pointer	8 bits	register number
2	immediate	operation size	value
3	immediate pointer	32 bits	pointer to memory location

Register numbers

value	register
0-31	r0-r31
32	rsp
33	resp
34	rfp

Condition codes

value	name	description
0	always	execute unconditionally
1	ifz	execute if zero flag is set
2	ifnz	execute if zero flag is not set
3	ifc	execute if carry flag is set
4	ifnc	execute if carry flag is not set
5	ifgt	execute if neither zero flag nor carry flag is set
6	iflteq	execute if zero flag or carry flag is set

Operation codes

value	name	operands	op sizes	description
0x00	NOP	none	8/16/32	no operation
0x01	ADD	src+tgt	8/16/32	add
0x02	MUL	src+tgt	8/16/32	multiply (unsigned)
0x03	AND	src+tgt	8/16/32	bitwise AND
0x04	SLA	src+tgt	8/16/32	shift left
0x05	SRA	src+tgt	8/16/32	shift right arithmetic (with sign extension)
0x06	BSE	src+tgt	8/16/32	bit set
0x07	CMP	src+tgt	8/16/32	compare
0x08	JMP	src	32	absolute jump
0x09	RJMP	src	32	relative jump
0x0A	PUSH	src	8/16/32	push value to stack
0x0B	IN	src+tgt	32	get input from I/O bus
0x0C	ISE	none	32	set interrupt enable flag
0x0D	MSE	none	32	set MMU enable flag
0x10	HALT	none	8/16/32	halt CPU
0x11	INC	src	8/16/32	increment (add 1)
0x13	OR	src+tgt	8/16/32	bitwise OR
0x14	IMUL	src+tgt	8/16/32	multiply (signed)
0x15	SRL	src+tgt	8/16/32	shift right logical (with zero extension)
0x16	BCL	src+tgt	8/16/32	bit clear
0x17	MOV	src+tgt	8/16/32	move value
0x18	CALL	src	32	absolute call
0x19	RCALL	src	32	relative call
0x1A	POP	src	8/16/32	pop value from stack
0x1B	OUT	src+tgt	32	output on I/O bus
0x1C	ICL	none	32	clear interrupt enable flag
0x1D	MCL	none	32	clear MMU enable flag
0x20	BRK	none	8/16/32	debug breakpoint
0x21	SUB	src+tgt	8/16/32	subtract
0x22	DIV	src+tgt	8/16/32	divide (unsigned)
0x23	XOR	src+tgt	8/16/32	bitwise XOR
0x24	ROL	src+tgt	8/16/32	rotate left
0x25	ROR	src+tgt	8/16/32	rotate right
0x26	BTS	src+tgt	8/16/32	test if bit set
0x27	MOVZ	src+tgt	8/16/32	move value and clear upper bits in target register
0x28	LOOP	src	32	absolute loop
0x29	RLOOP	src	32	relative loop
0x2A	RET	none	32	return from function
0x2C	INT	src	32	raise interrupt
0x2D	TLB	src	32	flush TLB and set page directory pointer
0x31	DEC	src	8/16/32	decrement (subtract 1)
0x32	REM	src+tgt	8/16/32	calculate remainder of division (unsigned)
0x33	NOT	src	8/16/32	bitwise NOT
0x34	IDIV	src+tgt	8/16/32	divide (signed)
0x35	IREM	src+tgt	8/16/32	remainder (signed)
0x39	RTA	src+tgt	32	calculate address relative to instruction pointer
0x3A	RETI	none	32	return from interrupt
0x3D	FLP	src	32	flush page from TLB

Operation sizes

value	description
0	byte (8 bits)
1	half (16 bits)
2	word (32 bits)
3	reserved

Interrupts and Exceptions

Interrupts indicate asynchronous hardware events (such as VSYNC) or execution of the int instruction, while exceptions indicate various synchronous errors.

There are 0x100 interrupt vectors and 5 exception vectors. Interrupt vectors are at 0x000 to 0x3FC, and exception vectors at 0x400 to 0x410. These memory locations simply store the address of the interrupt/exception handler, or 0x0 when no handler has been installed.

(TODO: what should the hardware do when a handler is missing?)

type	vector	description
interrupt	0 - 0xF0	free for software use
interrupt	0xFB	audio channel 3 refill
interrupt	0xFC	audio channel 2 refill
interrupt	0xFD	audio channel 1 refill
interrupt	0xFE	audio channel 0 refill
interrupt	0xFF	display VSYNC
exception	0x00	divide by zero
exception	0x01	invalid opcode
exception	0x02	page fault during read
exception	0x03	page fault during write
exception	0x04	breakpoint

Upon interrupt/exception entry, the CPU performs the following operations:

• read handler address from vector
• if swap sp is enabled:
  • switch to the exception stack pointer, and push the old stack pointer
• push current instruction pointer
• push flags (8 bits)
• push interrupt vector (0x00-0xff) or exception operand
• clear interrupt flag and swap sp flag
• jump to handler

Interrupt/exception handlers are exited through the reti instruction, which performs the following operations:

• pop and restore flags
• pop instruction pointer
• if swap sp flag is set, pop stack pointer

The flags are stored in the following format:

bit	description
0	zero flag
1	carry flag
2	interrupt flag
3	swap sp flag

MMU

If the MMU is enabled, two-level page tables are used to translate virtual addresses to physical addresses.

The address of the page directory can be set using the tlb instruction.

virtual address bits	purpose
11:0	lowest 12 bits of physical address
21:12	page table index
31:22	page directory index

Page directories and page tables are arrays of 1024 elements of the following format:

bits	bit mask ts	purpose
31:12	0xfffff000	address of page table or page
1	0x00000002	page is writable
0	0x00000001	page table or page is present

A page table walk is performed as follows:

• read page directory entry at page directory index in page directory
• abort if page table is not present
• read page table entry at page table index in page table
• abort of page is not present
• use physical page address and writability information from page table entry