70. LD – Load Indirect from Data Space to Register using Index X
70.1. Description
Loads one byte indirect from the data space to a register. For parts with SRAM, the data space consists
of the Register File, I/O memory, and internal SRAM (and external SRAM if applicable). For parts without
SRAM, the data space consists of the Register File only. In some parts the Flash Memory has been
mapped to the data space and can be read using this command. The EEPROM has a separate address
space.
The data location is pointed to by the X (16 bits) Pointer Register in the Register File. Memory access is
limited to the current data segment of 64KB. To access another data segment in devices with more than
64KB data space, the RAMPX in register in the I/O area has to be changed.
The X-pointer Register can either be left unchanged by the operation, or it can be post-incremented or
pre-decremented. These features are especially suited for accessing arrays, tables, and Stack Pointer
usage of the X-pointer Register. Note that only the low byte of the X-pointer is updated in devices with no
more than 256 bytes data space. For such devices, the high byte of the pointer is not used by this
instruction and can be used for other purposes. The RAMPX Register in the I/O area is updated in parts
with more than 64KB data space or more than 64KB Program memory, and the increment/decrement is
added to the entire 24-bit address on such devices.
Not all variants of this instruction is available in all devices. Refer to the device specific instruction set
summary.
In the Reduced Core tinyAVR the LD instruction can be used to achieve the same operation as LPM
since the program memory is mapped to the data memory space.
The result of these combinations is undefined:
LD r26, X+
LD r27, X+
LD r26, -X
LD r27, -X
Using the X-pointer:
Operation: Comment:
(i) Rd ← (X) X: Unchanged
(ii) Rd ← (X) X ← X + 1 X: Post incremented
(iii) X ← X - 1 Rd ← (X) X: Pre decremented
Syntax: Operands: Program Counter:
(i) LD Rd, X 0 ≤ d ≤ 31 PC ← PC + 1
(ii) LD Rd, X+ 0 ≤ d ≤ 31 PC ← PC + 1
(iii) LD Rd, -X 0 ≤ d ≤ 31 PC ← PC + 1
16-bit Opcode:
Atmel AVR Instruction Set Manual [OTHER]
Atmel-0856L-AVR-Instruction-Set-Manual_Other-11/2016
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