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Let us review the operation of the stack within the 68HC11, the stack is a defined area of RAM which is last in first out register (LIFO) . Access to the stack is made via a stack pointer (SP). In the 68HC11 there is only one stack pointer and it is initialised by the LDS command. To transfer data onto the stack we can use the command PSHA or PSHB i.e. push register A or B onto the stack. To remove data simply pull it i.e PULA or PULB, likewise the index registers may be stacked etc.
PSHA Push A onto Stack PSHB Push B onto Stack PSHX Push X onto Stack PSHY Push Y onto Stack PULA Pull A onto Stack PULB Pull B onto Stack PULX Pull X onto Stack PULY Pull Y onto Stack
Every time data is 'Pushed' onto the stack the Stack pointer is decrements accordingly. The stack is used often by the micro controller for temporary holding data, if a subroutine occurs, the processor stacks the return address and upon the receipt of the RTS command, it pulls the return address automatically. During an interrupt the processor stacks the full details of itself i.e.
SP Condition code SP-1 Accumulator A SP-2 Accumulator B SP-3 Index register X high SP-4 Index register X low SP-5 Index register Y high SP-6 Index register Y low SP-7 return address High SP-8 Return address Low SP-9 Stack pointer after interrupt
Therefore great care should be taken when using the stack as during an interrupt or subroutine if you place data on the stack you must remove the data before you return from it.
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