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The code segment below is to get the value of Ali) for an integer array A[N][M] for some constant values N and M. Assume the address of the array is in %rdx, the row is in %rdl and the column is in %rsi. shl $4, rdi lea ($rsi, &rdi, 4),rax mov ($rdx, rax, 4), teax Based on the code above, what is the number of columns (the value of M) in array A? You may give your answer in hex (starting with Ox) or in decimal.

The code segment below is to get the value of Ali) for an integer array A[N][M] for some constant values N and M. Assume the address of the array is in %rdx, the row is in %rdl and the column is in %rsi. shl $4, rdi lea ($rsi, &rdi, 4),rax mov ($rdx, rax, 4), teax Based on the code above, what is the number of columns (the value of M) in array A? You may give your answer in hex (starting with Ox) or in decimal.

Database System Concepts
Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
Section: Chapter Questions
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The code segment below is to get the value of A[i][j] for an **integer array** A[N][M] for some constant values N and M. Assume the address of the array is in `%rdx`, the row is in `%rdl` and the column is in `%rsi`. ``` shl $4, %rdi lea (%rsi,%rdi,4),%rax mov (%rdx,%rax,4), %eax ``` Based on the code above, what is the number of columns (the value of M) in array A? You may give your answer in hex (starting with 0x) or in decimal. ### Explanation - **shl $4, %rdi**: This line shifts the value in `%rdi` left by 4 bits. This is equivalent to multiplying the row index by 16, which suggests each row consists of a certain number of elements. - **lea (%rsi,%rdi,4),%rax**: This line loads the effective address by combining `%rsi` (the column index), the shifted `%rdi` (multiplied by 4, corresponding to the number of bytes per element in a row), into `%rax`. - **mov (%rdx,%rax,4), %eax**: This line moves the value from the calculated address in memory (base: `%rdx` plus offset: `%rax` multiplied by 4) into `%eax`. The shifting by 4 in the `shl` instruction indicates each element likely takes up 4 bytes, and loading the effective address considers both row and column offsets to access these bytes correctly. ### Number of Columns (Value of M) By interpreting the line instruction `lea (%rsi,%rdi,4),%rax`, where `%rdi` is multiplied by a factor related to column width (shift of 4 bits), you can deduce the size of each row and hence the number of columns (M). The computation in the code implies each row is accessed by an index multiplied by `M * 4`, meaning `M = 4`. Therefore, the number of columns M is 4.
Transcribed Image Text:The code segment below is to get the value of A[i][j] for an **integer array** A[N][M] for some constant values N and M. Assume the address of the array is in `%rdx`, the row is in `%rdl` and the column is in `%rsi`. ``` shl $4, %rdi lea (%rsi,%rdi,4),%rax mov (%rdx,%rax,4), %eax ``` Based on the code above, what is the number of columns (the value of M) in array A? You may give your answer in hex (starting with 0x) or in decimal. ### Explanation - **shl $4, %rdi**: This line shifts the value in `%rdi` left by 4 bits. This is equivalent to multiplying the row index by 16, which suggests each row consists of a certain number of elements. - **lea (%rsi,%rdi,4),%rax**: This line loads the effective address by combining `%rsi` (the column index), the shifted `%rdi` (multiplied by 4, corresponding to the number of bytes per element in a row), into `%rax`. - **mov (%rdx,%rax,4), %eax**: This line moves the value from the calculated address in memory (base: `%rdx` plus offset: `%rax` multiplied by 4) into `%eax`. The shifting by 4 in the `shl` instruction indicates each element likely takes up 4 bytes, and loading the effective address considers both row and column offsets to access these bytes correctly. ### Number of Columns (Value of M) By interpreting the line instruction `lea (%rsi,%rdi,4),%rax`, where `%rdi` is multiplied by a factor related to column width (shift of 4 bits), you can deduce the size of each row and hence the number of columns (M). The computation in the code implies each row is accessed by an index multiplied by `M * 4`, meaning `M = 4`. Therefore, the number of columns M is 4.
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