LA is used to
initialize the register specified by operand 1 with the address designated by
operand 2. Operand 2 may be expressed
using explicit notation ( see Explicit
Addressing) or symbolic notation, or a combination of both. Remember that each byte in memory is
numbered and that the number assigned to a byte is its address. The address of a field is the address of the
first byte of the field. Consider the
following example,
LA R9,AFIELD
The address of the fullword “AFIELD” , x’00001000’, is copied to
register 9, destroying the previous value in R9. The fullword is unchanged by this operation.
Since LA is an RX
instruction, an index register may be coded as part of operand 2 as in the
example below. We assume that register
6 is used as an index register and initially contains x’0000002F’. When the assembler processes the expression
AFIELD(R6), it uses the symbol AFIELD to determine a base register and a
displacement, leaving R6 as the index register. The address which is loaded into register 9 is the “effective address”
computed by adding the base register contents, plus the index register
contents, plus the displacement:
Effective address = C(Base register) + C(Index register) +
displacement
We assume that the contents
of the base register plus the displacement is x’00001000’. Then the effective address is x’00001000’ +
x’0000002F’ = x’0000102F’ .
LA R9,AFIELD(R6)
The example above uses a mixture of symbolic and explicit
addressing. The instruction could also
be coded using only explicit addresses:
LA R9,30(R7,R8)
In the example above assume
that R7 contains x’00001000’ and that R8 contains x’00000020’. R7 is treated as an index register, R8 is
the base register, and 30 is a displacement.
The effective address is C(R7) + C(R8) + 30 = x’00001000” + x’00000020’
+ x’0000001E’ = x’0000103E’. (Remember
that a decimal 30 is 1E in hexadecimal.)
After the instruction has executed, R9 contains x’0000103E’.
Some Unrelated Load Addresses
R4 = X’12121212’
R5 = X’00000008’
R6 = X’00000004’
Assume that AFIELD has address
x’00003000’.
AFIELD
DC F’4’ AFIELD = X’00000004’
LA R4,AFIELD R4 =
X’00003000’
LA R4,AFIELD(R6) R4 = X’00003004’
LA R4,AFIELD(R5) R4 = X’00003008’
LA R4,20(R5,R6) R4 =
X’00000020’ 4 + 8 + 20 = 32 = X’20’
Using R0 as an index indicates
that no index register is
desired:
LA R4,3(R0,R6) R4 =
X’00000007’ 4 + 3 = 7
Consider the next two
consecutively executed instructions.
LA R4,AFIELD R4 =
X’00003000’
LA R4,L’AFIELD(R0,R4) R4 =
x’00003004’
In the example above, the length
attribute (L’) is used as
a displacement
1. An old assembler joke:
Novice: What’s the
difference between a Load instruction and a Load Address
instruction?”
Old Hand: About a week - of debugging.
Seriously, you should pay attention when coding L or LA. Both instructions
compute the address of operand 2. In
the case of L, the machine retrieves the contents of the
fullword in memory at the specified address and places the four bytes in a
register. In the case of LA, the address is simply stored in a
register.
2. The LA instruction is
often used to change the location referenced by a DSECT:
TEST DSECT
TESTREC DS 0CL80
X DS ...
USING TEST,R5
LA R5,TABLE POINT AT TABLE AREA
...
LA R5,L’TESTREC(R0,R5) MOVE THE DSECT
(See DSECTs.)