120. ST (STD) – Store Indirect From Register to Data Space using Index Z
120.1. Description
Stores one byte indirect with or without displacement from a register to data space. 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. The EEPROM has
a separate address space.
The data location is pointed to by the Z (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 RAMPZ in register in the I/O area has to be changed.
The Z-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 Stack Pointer usage of the Z-pointer Register,
however because the Z-pointer Register can be used for indirect subroutine calls, indirect jumps and
table look-up, it is often more convenient to use the X- or Y-pointer as a dedicated Stack Pointer. Note
that only the low byte of the Z-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 RAMPZ 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/displacement 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.
The result of these combinations is undefined:
ST Z+, r30
ST Z+, r31
ST -Z, r30
ST -Z, r31
Using the Z-pointer:
Operation: Comment:
(i) (Z) ←Rr Z: Unchanged
(ii) (Z) ← Rr, Z ← Z+1 Z: Post incremented
(iii) Z ← Z - 1, (Z) ← Rr Z: Pre decremented
(iv) (Z+q) ← Rr Z: Unchanged, q:
Displacement
Syntax: Operands: Program Counter:
(i) ST Z, Rr 0 ≤ r ≤ 31 PC ← PC + 1
(ii) ST Z+, Rr 0 ≤ r ≤ 31 PC ← PC + 1
Atmel AVR Instruction Set Manual [OTHER]
Atmel-0856L-AVR-Instruction-Set-Manual_Other-11/2016
177
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