Register File

Register Reference

Name	Conventional Usage	Notes
r0	Routine Argument 1	Caller Saved
r1	Routine Argument 2	Caller Saved
r2	Routine Argument 3	Caller Saved
r3	Routine Argument 4	Caller Saved
r4	Upper Accumulator	Callee Saved
r5	Accumulator	Callee Saved
r6	General Purpose	Callee Saved
r7	General Purpose	Callee Saved
r8	General Purpose	Callee Saved
r9	General Purpose	Caller Saved
ra	Memory Access Upper	Caller Saved
rb	Memory Access Lower	Caller Saved
rih	Interrupt Address Upper	Location of interrupt routine
ril	Interrupt Address Lower	Location of interrupt routine
rsh	Stack Pointer Upper	Tracked with push/pop instructions
rsl	Stack Pointer Lower	Tracked with push/pop instructions
rfl	Condition codes	Cannot be directly used, affected by other operations

General Purpose Registers

• 8-bit word size
• Any of them can be used together with adjacent registers for address operations
• Can be used for calculations without additional side effects
• 5 registers (r4 through r8) available under all circumstances

Special Registers

Device Select

• Used for memory operations, can be used as a general register when not accessing memory
  • TODO: only by convention or a hard requirement?
• When memory operations occur, this can select the device/memory bank
• Effectively allows for 256 different I/O devices with 16-bit address spaces

Memory Access

• Conventionally stores any 16-bit address in ra and rb

Interrupt Address

• Can be used as a general register if interrupts aren't enabled
• rih and ril make up a 16-bit address pointing to the desired interrupt routine

Stack Pointer Registers

• rsh and rsl make up a 16-bit address pointing to the current descending stack address.
• Modified indirectly with push and pop instructions
• Can optionally be used as a general purpose register with severe side effects

Flag register

• Stores various flags
• Cannot be used as a general register under any circumstance

Bit	Name	Description	Affected By
0	c	Carry flag, final carry output result	cp, ALU functions
1	z	Zero flag, indicates result of an operation is zero	cp
2	s	Sign flag, MSB of the result	cp, ALU functions
3	p	Parity flag, LSB of the result	cp, ALU functions
4	o	Overflow flag, indicates whether operation overflowed	cp, ALU functions
5
6	i	Interrupt enable	ei, di
7	w	CPU in wait mode (paused execution)	wait