3.4
Application Notes
3.4.1
Notes on Stack Area Use
When word data is accessed in the H8/3887 Series, the least significant bit of the address is
regarded as 0. Access to the stack always takes place in word size, so the stack pointer (SP: R7)
should never indicate an odd address. Use PUSH Rn (MOV.W Rn, @–SP) or POP Rn (MOV.W
@SP+, Rn) to save or restore register values.
Setting an odd address in SP may cause a program to crash. An example is shown in figure 3-6.
→
PCH
PCL
SP
R1L
PCL
H'FEFC
H'FEFD
→
SP
→
SP
H'FEFF
MOV. B R1L, @–R7
Stack accessed beyond SP Contents of PCH are lost
BSR instruction
SP set to H'FEFF
Notation:
PCH: Upper byte of program counter
PCL: Lower byte of program counter
R1L: General register R1L
SP: Stack pointer
Figure 3-6 Operation when Odd Address is Set in SP
When CCR contents are saved to the stack during interrupt exception handling or restored when
RTE is executed, this also takes place in word size. Both the upper and lower bytes of word data
are saved to the stack; on return, the even address contents are restored to CCR while the odd
address contents are ignored.
85
ETC [ ETC ]
