LM is used to copy a set of consecutive fullwords in main storage into a “consecutive” collection of registers. The collection of registers is specified by coding the beginning and ending registers as the first two operands. For instance,

LM R5,R8,XWORDS

would be used to load registers 5, 6, 7, and 8. If the second operand specifies a lower register than the first operand, then a “wrap-around” occurs. For instance,

LM R14,R12,12(R13)

would be used to load registers 14, 15, 0, 1, ..., 12 - every register except 13.

The registers are loaded starting with the fullword at the address specified in the third operand. In the first example above, register 5 would be loaded with the fullword at XWORDS, register 6 with the fullword at XWORDS+4, register 7 with the fullword at XWORDS+8, and register 8 with the fullword at XWORDS+12. In the second example, register 14 is loaded with the fullword at the explicit address indicated as a 12 byte displacement off register 13. Register 15 is loaded with the fullword at a 16 byte displacement off register 13. Register 0 is loaded with the fullword at a 20 byte displacement off register 13. The remaining registers are loaded in a similar manner.

Here is an illustrated example.

STM R7,R9,REGWORDS

In the example above, the consecutive range of registers ( 7, 8, and 9 ) are loaded with three consecutive fullwords in memory starting with the fullword specified as REGWORDS.

Some Unrelated LM’s

XWORD DC F’200’,F’50’,F’30’,F’90’

...

LM R5,R6,XWORD R5 = F’200’

R6 = F’50’

LM R15,R1,XWORD R15 = F’200’

R0 = F’50’

R1 = F’30’

LM R4,R7,XWORD R4 = F’200’

R5 = F’50’

R6 = F’30’

R7 = F’90’

LM R14,R12,12(R13) THIS LOADS ALL REGISTERS EXCEPT 13

STARTING WITH THE FULLWORD THAT

IS 12 BYTES OFF REGISTER 13. (SEE

PROGRAM LINKAGE.)

1) This instruction is helpful for creating subroutines. Branch to a subroutine and immediately save the registers with STM. This allows your subroutine to freely use the registers. Before exiting your subroutine, restore the registers with LM. Create a register “save area” for each subroutine.

BAS R6,SUBRTN CALL THE SUBROUTINE

...

SUBRTN EQU *

STM R14,R12,SUBSAVE SAVE THE REGISTERS

... (SUBROUTINE CODE)

LM R14,R12,SUBSAVE RESTORE THE REGISTERS

BR R6 BRANCH BACK TO CALLER

SUBSAVE DS 15F FIFTEEN FULLWORDS FOR THE REGISTERS

2) Read about the role that LM plays in implementing the Program Linkage conventions.