Rainbow Electronics ATtiny12 - Manual (Page 12)

12
ATtiny11/12
1006C–09/01
Relative Program Addressing,
RJMP and RCALL Figure 12. Relative Program Memory Addressing
Program execution continues at address PC + k + 1. The relative address k is -2048 to
2047.
Constant Addressing Using
the LPM Instruction Figure 13. Code Memory Constant Addressing
Constant byte address is specified by the Z-register contents. The 15 MSBs select word
address (0 - 511), the LSB selects low byte if cleared (LSB = 0) or high byte if set
(LSB = 1).
Subroutine and Interrupt
Hardware Stack The ATtiny11/12 uses a 3-level-deep hardware stack for subroutines and interrupts. The
hardware stack is 9 bits wide and stores the program counter (PC) return address while
subroutines and interrupts are executed.
RCALL instructions and interrupts push the PC return address onto stack level 0, and
the data in the other stack levels 1-2 are pushed one level deeper in the stack. When a
RET or RETI instruction is executed the returning PC is fetched from stack level 0, and
the data in the other stack levels 1-2 are popped one level in the stack.
If more than three subsequent subroutine calls or interrupts are executed, the first val-
ues written to the stack are overwritten. Pushing four return addresses A1, A2, A3, and
A4, followed by four subroutine or interrupt returns, will pop A4, A3, A2, and once more
A2 from the hardware stack.
+1
$1FF
$000
PROGRAM MEMORY
15 1 0
Z-REGISTER