Data Exchanges
MOV, PUSH, and POP opcodes all involve copying the data found in the source address to the destination address; the original data in the source is not changed. Exchange instructions actually move data in two directions: from source to destination and from destination to source. All addressing modes except immediate may be used in the XCH (exchange) opcodes:
|
Mnemonic |
Operation |
|
XCH A,Rr |
Exchange data bytes between register Rr and A |
|
XCH A,add |
Exchange data bytes between add and A |
|
XCH A,@Rp |
Exchange data bytes between A and address in Rp |
|
XCHD A,@Rp |
Exchange lower nibble between A and address in Rp |
Exchanges between A and any port location copy the data on the port pins to A, while the data in A is copied to the port latch. Register A is used for so many instructions that the XCH opcode provides a very convenient way to "save" the contents of A without the necessity of using a PUSH opcode and then a POP opcode.
The following table shows examples of data moves using exchange opcodes:
|
Mnemonic |
Operation |
|
XCH A,R7 |
Exchange bytes between register A and register R7 |
|
XCH A,0F0h |
Exchange bytes between register A and register B |
|
XCH A,@R1 |
Exchange bytes between register A and address in R 1 |
|
XCHD A,@R1 |
Exchange lower nibble in A and the address in R 1 |
CAUTION
All exchanges are internal to the 8051.
All exchanges use register A.
When using XCHD, the upper nibble of A and the upper nibble of the address location in Rp do not change.
This section concludes the listing of the various data moving instructions; the remaining sections will concentrate on using these opcodes to write short programs.