10.3.2 Memory Access

For read/write operations, the Flash memory can be accessed from either the code space or the CPU data space. The Flash is accessible through the LPM and SPM instructions when using the code space.

Additionally, when accessed through the CPU data space, the Flash memory is byte accessible, meaning that it shares the same address space and access instructions as SRAM, EEPROM and I/O registers, and it is accessible using LD/ST instructions.

For the LPM and SPM instructions, address 0x0000 is the start of the Flash, but for LD and ST, it is 0x8000, as shown in the Memory Map section.

Addressing Flash Memory in Code Space

For read and write access to the Flash memory in the code space, the RAMPZ register is concatenated with the Z register to create the Address Pointer used for LPM/SPM access.

The Flash is word-accessed and organized in pages, so the Address Pointer can be treated as having two sections, as shown in the figure above. The word address in the page (FWORD) is held by the Least Significant bits (LSbs) in the Address Pointer, while the Most Significant bits (MSbs) in the Address Pointer hold the Flash page address (FPAGE). Together, FWORD and FPAGE hold an absolute address to a word in the Flash.

The Flash is word-accessed for code space write operations, so the Least Significant bit (bit 0) in the Address Pointer is ignored.

For Flash read operations, one byte is read at a time. The Least Significant bit (bit 0) in the Address Pointer is used to select the low or high byte in the word address for this. If this bit is ‘0’, the low byte is read, and if this bit is ‘1’, the high byte is read.

While initiating a programming operation, the address is latched, and the Address Pointer can be updated and used for other operations.

Addressing Flash in CPU Data Space

The CPU data space is limited to 32 KB. For devices with a Flash memory size greater than 32 KB, the Flash memory is divided into blocks of 32 KB. Those blocks are mapped into data space using the FLMAP bit field of the Control B (CTRLB) register.

For read and write access to the Flash memory in the CPU data space, the LD/ST instructions are used to access one byte at a time.