I came across something strange while reading the datasheet for the microcontroller in the Arduino Uno board.
It is the (8) note in pg.296-297 of this datasheet: what does WORDS mean in that context?
The notation is being used to indicate that the 16-bit address (as formed by the central bytes of a Write Program Memory Page instruction) is the address of a program-memory word, rather than a program-memory byte.
The size of program memory is specified in bytes, but it often is addressed as words. For example, the PC (program counter) register contains word addresses.
For some instructions, when the Z register is used to address program memory, it contains a word number and a byte selector:
The Z-register in the Register File is used to access the Program memory. This 16 bits register pair is used as a 16 bits pointer to the Program memory. The 15 most significant bits selects the word address in Program memory. Because of this, the word address is multiplied by two before it is put in the Z-register. ... The least significant bit of the Z Address Register selects either Low byte (0) or High byte(1) of the Program memory word. — AVR108: Setup and Use of the LPM Instruction, page 1
Note: Some of the low bits of the word address in a “Write Program memory page” serial programming instruction are treated as don't-cares. The number of don't-care bits depends on flash page size, which in Atmel AVR devices is 32, 64 or 128 words, depending on device series.
For example (as shown in Table 129 on page 304 of doc2467.pdf) the ATmega128 with 128-word (256-byte) flash pages has seven don't-care bits in the low byte of the address.
Table 129. SPI Serial Programming Instruction Set -----------Instruction Format---------- Instruction Byte 1 Byte 2 Byte 3 Byte 4 Operation Write Program 0100 1100 aaaa aaaa bxxx xxxx xxxx xxxx Memory Page Write Program Memory Page at address a:b.
See jwpat7's answer.
My answer is correct re address word BUT
his points out that "Words" are being addressed and not bytes. While my answer is not wrong, his is more likely to be what the writer meant in his excessively cryptic footnote.
A Byte is most usually used to refer to an 8 bit quantity.
A "word" is usually used to refer to a multi-Byte variable with the length depending on both processor type and context.
In this case, on page 296, table 28-19 information is given on the format of 4 byte Instructions. These instructions are NOT referred to as Words although in some other contexts they might be.
However, the "Write Program memory page" instruction contains the address of a memory page. This address is made up of a 16 bit field comprised of 2 Bytes - an MSB and an LSB.
The notation "Words" is shown against the MSB and the LSB to denote (not very clearly) that a single 16 bit "Address Word"is formed by the MSB + LSB combination.