XA User Guide 6-176 4/17/98
6.6 Summary Of Illegal Operand Combinations On The XA
All but one case are instructions that specify or imply 2 write operations to a single register file
location within a single instruction. The other case is a possible corruption of the source register
data by an auto-increment before it is read. These conditions are not detected by XA hardware. The
instruction/operand combinations indicated should not be used when writing XA code.
NOTES:
1 This addressing mode is illegal when the source and destination are the same register. This would cause
both a data write and an auto-increment operation to the same register.
2 This instruction is illegal when the source and destination pointer registers are the same register. This
would cause two auto-increment operations to the same register.
3 This instruction is illegal when the source and destination are the same register. The source register would
be auto-incremented and read at the same time, with an undefined result.
4 This instruction is illegal when the source and destination are the same register. This would cause two
writes to the same register.
5 This addressing mode is illegal when the indirect address of the destination points to the pointer register
itself in the register file. This is possible only when 8051 compatibility mode is enabled. This would
cause both a data write and an auto-increment operation to the same register.
6 This instruction is illegal when the indirect address of the source operand points to the destination register
itself in the register file. This is possible only when 8051 compatibility mode is enabled. This would
cause two writes to the same register.
7 This instruction is illegal when the direct address of the source operand points to the destination register
itself in the register file. This is possible only when 8051 compatibility mode is enabled. This would
cause two writes to the same register.
8 A POP to R7 (the stack pointer) would cause both a data write and an auto-increment operation to the
same register.
Instruction(s) affected Reason for illegal combination
(any op) Rx, [Rx+] Auto-increment plus explicit write
1
mov [Rx+], [Rx+] Double auto-increment of one register
2
(any op) [Rx+], Rx Auto-increment write may corrupt the source register before it is read
3
NORM Rx, Rx Result and shift count stored in the same register
4
XCH Rx, Rx Double write of a single register
4
(any op) [Rx+], Ry Auto-increment plus indirect write to same register
5
(any op) [Rx+], [Ry+] Auto-increment plus indirect write to same register
5
(any op) [Rx+], #data Auto-increment plus indirect write to same register
5
XCH Rx, [Rx] Indirect write plus explicit write to the same register
6
XCH Rx, direct Direct write plus explicit write to the same register
7
POP R7 Stack pointer auto-increment plus explicit write to R7/SP
8
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