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Load and store instruction formats used to access array data

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• Introduction
  • We have seen the basic form of the load/store instruction of the ARM processor in these webpages ( load and store)

    I repeat the (summary of the) material here for review (and reminder).

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  • Syntax and meaning of the basic form of the load register instruction is:

    Syntax Meaning of the instruction ---------------- -------------------------------------------------- ldr rN, [rM] Load 4 bytes from memory at the address given in rM into the (destination) register rN The 4 bytes are stored in the register as follows: ldrsh rN, [rM] Load 2 bytes from memory at the address given in rM into the (destination) register rN AND fill the left half of register with the sign bit The 2 bytes are stored at the right and then sign extended to 32 bits: ldrsb rN, [rM] Load 1 byte from memory at the address given in rM into the (destination) register rN AND fill the left 3/4 of register with the sign bit The byte is stored at the right and then sign extended to 32 bits:

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  • Syntax and meaning of the basic form of the store register instruction

    Syntax Meaning of the instruction ---------------- -------------------------------------------------- str rN, [rM] Store 4 bytes from register rN to memory (consecutively) at the address given in register rM The 4 bytes are taken from the register as follows: strh rN, [rM] Store 2 bytes from register rN to memory (consecutively) at the address given in register rM The 2 bytes are taken from the register as follows: strb rN, [rM] Store 1 byte from register rN to memory (consecutively) at the address given in register rM The byte is taken from the register as follows:

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    Although the basic form is sufficient to access simple variable, it is not adequate when we need to compute the address of an array element using:

    address of array element A[i] = base address (A) + i×size(one array element)

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• The base + offset mode of the load instruction
  • The load instruction has a more advanced form:

    Syntax and meaning of the base+offset form of the load register instruction is:

    Syntax Meaning of the instruction ---------------- -------------------------------------------------- ldr rN, [rM,off] Load 4 bytes from memory at the address given by rM+off into the (destination) register rN The 4 bytes are stored in the register as follows: ldrsh rN, [rM,off] Load 2 bytes from memory at the address given by rM+off into the (destination) register rN AND fill the left half of register with the sign bit The 2 bytes are stored at the right and then sign extended to 32 bits: ldrsb rN, [rM,off] Load 1 byte from memory at the address given by rM+off into the (destination) register rN AND fill the left 3/4 of register with the sign bit The byte is stored at the right and then sign extended to 32 bits:

    For offset, you can use:

    A constant value #n The base+offset load instruction syntax is then: ldr rN, [rM, #n] // Effective address = rM + n ldrsh rN, [rM, #n] ldrsb rN, [rM, #n]      or A value stored in a register The base+offset load instruction syntax is then: ldr rN, [rM, rX] // Effective address = rM + rX ldrsh rN, [rM, rX] ldrsb rN, [rM, rX]

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    How to use the base + offset form of the load instruction to access array elements:

    Move the base address of the array into some register (e.g.: rx) If the array index is a constant: Pre-compute the offset (because the index is a constant, the offset will also be constant) Let the pre-computed offset be equal to n use ldr ..., [rx,#n],   ldrsh ..., [rx,#n]   or   ldrsb ..., [rx,#n] to access the array element depending on the data type of the array If the array index is not a constant: Compute the offset and store the result in another register (e.g.: ry) Then use ldr ..., [rx,ry],   ldrsh ..., [rx,ry]   or   ldrsb ..., [rx,ry] to access the array element depending on the data type of the array

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• The base + offset mode of the store instruction
  • The store instruction also has a more advanced form:

    Syntax and meaning of the base+offset form of the store register instruction is:

    Syntax Meaning of the instruction ---------------- -------------------------------------------------- str rN, [rM,off] Store 4 bytes from register rN to memory (consecutively) at the address given by the sum rM+off The 4 bytes are taken from the register as follows: strh rN, [rM,off] Store 2 bytes from register rN to memory (consecutively) at the address given by the sum rM+off The 2 bytes are taken from the register as follows: strb rN, [rM,off] Store 1 byte from register rN to memory (consecutively) at the address given by the sum rM+off The byte is taken from the register as follows:

    For offset, you can use:

    A constant value #n The base+offset store instruction syntax is then: str rN, [rM, #n] // Effective address = rM + n strsh rN, [rM, #n] strsb rN, [rM, #n]      or A value stored in a register The base+offset store instruction syntax is then: str rN, [rM, rX] // Effective address = rM + rX strsh rN, [rM, rX] strsb rN, [rM, rX]

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    How to use the base + offset form of the store instruction to update an array element:

    Move the base address of the array into some register (e.g.: rx) If the array index is a constant: Pre-compute the offset (because the index is a constant, the offset will also be constant) Let the pre-computed offset be equal to n use str ..., [rx,#n],   strh ..., [rx,#n]   or   strb ..., [rx,#n] to update the array element depending on the data type of the array If the array index is not a constant: Compute the offset and store the result in another register (e.g.: ry) Then use str ..., [rx,ry],   strh ..., [rx,ry]   or   strb ..., [rx,ry] to update the array element depending on the data type of the array