Memory management and registers - kammyfur/vixen GitHub Wiki

The Vixen architecture contains 2 types of memory: registers and virtual memory. Registers are used for immediate processing within the CPU itself, and virtual memory is used for the rest.

Vixen currently supports between 69 megabytes (72352256 bytes) and 4 gigabytes (4294967295 bytes) of virtual memory. This amount can be changed at the hardware level.

The following table lists all the registers, the internal ID used to refer to them, how large they are, and whether they are readable and/or writable.

Name	Internal ID	Size	Description	Readable	Writable
r0	0001	32 bits	General-purpose	x	x
r1	0011	32 bits	General-purpose	x	x
r2	0012	32 bits	General-purpose	x	x
r3	1000	32 bits	General-purpose	x	x
r4	1001	32 bits	General-purpose	x	x
r5	1002	32 bits	General-purpose	x	x
r6	1003	32 bits	General-purpose	x	x
r7	1004	32 bits	General-purpose	x	x
r8	1005	32 bits	General-purpose	x	x
r9	1006	32 bits	General-purpose	x	x
r10	1007	32 bits	General-purpose	x	x
r11	1008	32 bits	General-purpose	x	x
r12	1009	32 bits	General-purpose	x	x
r13	100a	32 bits	General-purpose	x	x
r14	100b	32 bits	General-purpose	x	x
pc	program_counter	32 bits	Address of current instruction in memory	x	x
sr	system_register	8 bits	Internal processor flags	*	*
up	stack_pointer	32 bits	Pointer to the top of the user stack	*	*
ss	system_stack	-	Internal stack used for interrupt handling and subroutines
sp	-	-	Pointer to the top of the system stack

* There are instructions that might allow for reading/writing to these registers, but they cannot be directly read from/written to.

Since Vixen is using virtual memory, all the memory the CPU sees may not necessarily correspond to system RAM. Not all memory regions are writable, and they are all readable.

Start	End	Size	Name	Description
00000000	000001ff	512B	Specification	The CPU specification region contains read-only information about the CPU, such as its model number, clock speed, bus speed, supported extensions, bus width, address with, and more.
00000200	040001ff	64M	Boot ROM	ROM contains the system's basic code, which is usually used to load another external program (bootloader). It is usually stored in a dedicated chip and can (in most cases) not be written to.
04000200	041001ff	1M	System Bus	This I/O region is used to access the system bus and send or receive information from/to external devices such as terminals, non-volative memory, sound cards or printers.
04100200	045001ff	4M	User Stack	The user stack is used for various stack-related operations. It can be used through the psh and pll instructions (for example) without conflict with the CPU itself.
04500200	04500207	8B	Interrupt handlers	This section of memory is used to store pointers to (respectively) an interrupt handler and a double fault handler.
04500208	-	-	RAM	RAM is used to store disorganized memory. It is slower than registers but used to off-load data that does not need immediate processing.

Only memory from 04000200 is natively writeable. Bus devices may also request for their memory to be locked. RAM is dynamically allocated based on the hardware.

There are multiple ways one can read or write data. This mode is defined in nibble 0 (the last 4 bits when reading left to right) when encoding operation codes:

Operation code	Name	Description	Assembly syntax
0	Immediate	Uses literal values.	#<number> #'<char>'
1	Direct	Uses a register within the CPU.	<name>
2	Absolute (Indirect)	Uses a pointer stored in the given memory location	[<addr>]
3	Absolute	Uses an absolute memory location starting from 00000000	<addr>
4	Relative	Uses a memory location relative to the current location (program counter)	+<offset> -<offset>
5	Implied	Uses no operands or implicit operands	-
6*	Absolute (Indexed)	Uses an absolute memory location added with the literal value	[<addr> + #<index>] [<addr> - #<index>]
7	Absolute (Reg Indirect)	Uses a pointer stored in the given register	[<name>]
8*	Absolute (Mem Indexed)	Uses an absolute memory location added with another memory location	[<addr> + <addr>] [<addr> - <addr>]
9*	Absolute (Reg Indexed)	Uses an absolute memory location added with a register	[<addr> + <name>] [<addr> - <name>]

Proposed change:

Operation code	Name	Description	Assembly syntax
0	Immediate	Uses literal values.	<number> '<char>'
1	Direct	Uses a register within the CPU.	<name>
2	Absolute (Indirect)	Uses a pointer stored in the given memory location	[*<addr>]
3	Absolute	Uses an absolute memory location starting from 00000000	[<addr>]
4	Relative	Uses a memory location relative to the current location (program counter)	+<offset> -<offset>
5	Implied	Uses no operands or implicit operands	-
6*	Absolute (Indexed)	Uses an absolute memory location added with the literal value	[<addr> + <index>] [<addr> - #<index>]
7	Absolute (Reg Indirect)	Uses a pointer stored in the given register	[<name>]
8*	Absolute (Mem Indexed)	Uses an absolute memory location added with another memory location	[<addr> + *<addr>] [<addr> - *<addr>]
9*	Absolute (Reg Indexed)	Uses an absolute memory location added with a register	[<addr> + <name>] [<addr> - <name>]

In assembly code, numbers can be in different formats:

Syntax	Base	Example
%	Binary (base 2)	%10101010
$	Hexadecimal (base 16)	$00ab16ef
-	Decimal (base 10)	123

Proposed change:

Syntax	Base	Example
b	Binary (base 2)	10101010b
h	Hexadecimal (base 16)	00ab16efh
o	Octal (base 8)	00ab16efo
-	Decimal (base 10)	123