Assume X-$2ABI, Y-SACBF, SP = $CE06, A-SEA, B = $FB and CCR $0F a) The contents

ID: 3349521 • Letter: A

Question

Assume X-$2ABI, Y-SACBF, SP = $CE06, A-SEA, B = $FB and CCR $0F a) The contents of memory are shown in the table below. RTI instruction at the end of the ISR is executed, that is, ; part of the code ISR ; interrupt service routine part of the ISR code PULA; part of the ISR code RTI; being executed now What are the contents of each CPU register (PC, X, Y, CCR, SP, A and B) in the 68HC12 just after the RTI has been executed? MEMORY Address Data E03 E04 EO5 E06 E07 E08 E09 EOA EOB EOC EOD EOE EOF E10 12 A3 4E 59 EO SP 2D 4E 5F 80 PC =

Explanation / Answer

When an ISR is executed the sequence in which the status registers are stored in Stack is

Content

Address

CCR

SP - 1

SP - 2

SP - 3

SP - 5

Return address

SP - 7

When RTI is executed the contents are retrieved in the same order. A, B, CCR are 8 bit registers and X, Y, PC are 16 bit registers.

CCR = $12

A = $A3

B = $4E

X = $59E0

Y = $00C1

PC = $2D4E

SP = $CE0F