Hardware manual

Rev. 3.0, 09/98, page 76 of 361

4.4 Note on Stack Handling

In word access, the least significant bit of the address is always assumed to be 0. The stack is

always accessed by word access. Care should be taken to keep an even value in the stack pointer

(general register R7). Use the PUSH and POP (or MOV.W Rn, @−SP and MOV.W @SP+, Rn)

instructions to push and pop registers on the stack.

Setting the stack pointer to an odd value can cause programs to crash. Figure 4.8 shows an

example of damage caused when the stack pointer contains an odd address.

Legend:

PCH

PCL

R1L

: Upper byte of program counter

: Lower byte of program counter

: General register

: Stack pointer

H'FECC

H'FECD

H'FECF

R1L

MOV.B R1L, @-R7BSR instruction

H'FECF set in SP PC is improperly stored

beyond top of stack

PCH is lost

Figure 4.8 Example of Damage Caused by Setting an Odd Address in R7

Although the CCR consists of only one byte, it is treated as word data when pushed on the stack.

In the hardware interrupt exception-handling sequence, two identical CCR bytes are pushed onto

the stack to make a complete word. When popped from the stack by an RTE instruction, the CCR

is loaded from the byte stored at the even address. The byte stored at the odd address is ignored.