# 360 Assembly/360 Instructions/LH

LH - Load Halfword - Opcode 48 - RX format Instruction (32 Bit)

## Format

    LH 2,N2
    LH 11,106(8,10)
The specific syntax is
    LH target register,offset(index register,base register)
Where the offset, index register and base register values will be determined automatically by the assembler if the USING pseudo-instruction has been used, and the target address label used is within 4096 bytes of the value of some base register.

## Opcode

RX Instruction (4 bytes) :
| | Byte 1 | Byte 2 | Byte 2 | Bytes 3 and 4 | Bytes 3 and 4 | Bytes 3 and 4 |
| | Byte 1 | target register | Source Address | Source Address | Source Address | Source Address |
| (In Hex) | (8 bits) Opcode 48 | (4 bits) 0..F | (4 bits) index register 0..F | (4 bits) base register 0..F | (12 bits) displacement 0..FFF | (12 bits) displacement 0..FFF |
* The first argument is a target register which value is affected by the instruction.
* The second argument is the source value location address where the contents are to be loaded into the target register. This consists of a base register, an index register, and a displacement address. The displacement is used as an unsigned offset of 0 to 4095 from the value in the base register, and this displacement is added to the value contained in the base register. If the index register is 0, it is ignored; otherwise, its contents are added to the previous result to obtain the final address.
* The base_register and index_register values are 0 to 15. The offset address is 0 to 4095.
The target and base_register values will be determined automatically by the assembler if the USING pseudo-instruction has been used, and the target address label used (the displacement) is within 4096 bytes of the value of some base register.

## Availability

The LH instruction is available on all models of the 360, 370 and z/System.

## Operation

The LH instruction reads a 16-bit integer value from the memory address specified by the argument and puts it in the low 16 bits of the target register specified by the first argument. The left half of the register is replaced by zeros or ones to reflect the sign (leftmost bit) of the halfword.
The source argument address must be aligned to a half-word boundary (i.e. 2 bytes) or an exception will occur. The Condition Code field in the Program Status Word is not changed.

## Exceptions and Faults

* The source address must be within the range of valid memory or an operation exception occurs.
* The source address must be aligned to a halfword boundary or an address exception occurs.
* The storage key for the source address must be the same as the current process (or the process must be privileged with a key of zero) or a memory protect violate exception occurs.

## Most Closely-related Instruction

The most closely-related instruction is Add Logical Register (ALR) and not Load Address (LA), as is sometimes assumed. Whereas L loads a value from memory, LA does not access memory; it only calculates a number, which can but does not have to be an address.

### 32-Bit Instructions

* To load an 8-bit byte (character) value from memory, see IC.
* To load one or more 8-bit byte values into a register according to a mask, see ICM
* To load a 16-bit half-word value from memory, see LH.
* To load a 12-bit unsigned value into a register or add up to 4095 to the values in one or two 32-bit registers, see LA.
* To copy a 32-bit word value from one register to another register, see LR.
* To load a 32-bit word value from memory, see L.
* To store an 8-bit byte (character) value in memory, see STC.
* To store one or more 8-bit byte values into memory according to a mask, see STCM
* To store a 16-bit half-word value in memory, see STH.
* To store a 32-bit word value in memory, see ST.

### 64-Bit Instructions

* To load a 20-bit signed value into a 32-bit or 64-bit register from an extended (20-bit) signed memory offset, see LAY.
* To load a 32-bit value into a 64-bit register from an extended (20-bit) signed memory offset, see LGF.
* To copy a 64-bit value from one 64-bit register to another 64-bit register, see LGR.
* To load a 64-bit value into a 64-bit register from memory, see LG.
* To store a 32-bit value from a register to an extended (20-bit) signed memory offset, see STY.
* To store a 64-bit value from a 64-bit register to an address in memory, see STG.
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