# A 2D array [Courses](https://asm-editor.specy.app/learn/courses.md) › [M68K assembly](https://asm-editor.specy.app/learn/courses/m68k.md) › Examples Web page: https://asm-editor.specy.app/learn/courses/m68k/examples/two-dimensional-array > Three rows of four words, one element reached by row and column, and a whole column added up by stepping a row at a time. The same program in: [MIPS](https://asm-editor.specy.app/learn/courses/mips/examples/two-dimensional-array.md), [RISC-V](https://asm-editor.specy.app/learn/courses/risc-v/examples/two-dimensional-array.md), [Z80](https://asm-editor.specy.app/learn/courses/z80/examples/two-dimensional-array.md), [x86-64](https://asm-editor.specy.app/learn/courses/x86/examples/two-dimensional-array.md) A grid in memory is still a line of values. This example stores three rows of four words at `$2000`. It reads one value using a row and column, then adds the three values in that column. Both row and column indices start at **0**: the rows are 0, 1 and 2, and the columns are 0, 1, 2 and 3. ```m68k ROWS equ 3 COLS equ 4 lea grid, a0 move.l #2, d0 ; row 2, the third row move.l #1, d1 ; column 1, the second column move.l d0, d2 mulu #COLS, d2 ; skip row * COLS words add.l d1, d2 ; then skip col more words add.l d2, d2 ; convert words to bytes moveq #0, d3 move.w (a0, d2), d3 ; read grid[row][col] lea grid, a1 move.l d1, d4 add.l d4, d4 ; col * 2 bytes add.l d4, a1 ; address of grid[0][col] clr.l d5 ; sum starts at 0 move.l #ROWS-1, d6 ; dbra runs ROWS times column: add.w (a1), d5 ; add this row's word add.l #COLS*2, a1 ; advance by one whole row dbra d6, column org $2000 grid: dc.w 1, 2, 3, 4 dc.w 10, 20, 30, 40 dc.w 100, 200, 300, 400 ``` Each `dc.w` line shows a row, but the twelve words occupy consecutive addresses from `$2000` through `$2017`. To find `grid[row][col]`, skip `row * COLS + col` words from the start. For row 2, column 1, that is `2 * 4 + 1 = 9` words. Each word takes two bytes, so the byte offset is 18 (`$12`). Starting at `$2000`, `(a0, d2)` reads the word at `$2012`: 200. `moveq #0, d3` clears the whole register first; `move.w` then writes only its low 16 bits. After the read, `d3` is `000000C8`. For the column sum, `a1` starts at `$2002`, the address of row 0, column 1. The words below it are at `$200A` and `$2012`: each next row starts `COLS * 2 = 8` bytes later. The loop reads 2, 20 and 200, so the low word of `d5` becomes 222 (`$00DE`). `clr.l d5` makes the upper word zero here. The additions use `add.w`, though, so a larger sum that exceeds 16 bits would wrap in the low word; it would not carry into the upper word. `dbra` uses the low word of `d6`. Starting it at `ROWS-1`, or 2, gives three passes. Select **Build**, then **Run**. Check `d3 = 000000C8` and `d5 = 000000DE` in the registers panel. Select **Build** again to reset, then use **Step** to follow `a1` through `$2002`, `$200A` and `$2012` as the three words are read. The final advance leaves `a1` at `$201A`, outside the grid, but the loop ends without reading there. ## Try another cell and column Change `#2` in `d0` to `#1`, and `#1` in `d1` to `#3`. Before selecting **Build** and **Run**, predict the address and value read into `d3`, and the final sum in `d5`.
Show answer Row 1, column 3 is word `1 * 4 + 3 = 7`, at `$2000 + 7 * 2 = $200E`. Its value is 40, so `d3 = 00000028`. The column contains 4, 40 and 400; their sum is 444, so `d5 = 000001BC`.
Use row indices 0 through 2 and column indices 0 through 3. The program has no bounds check: column 4 with row 0 calculates the address of row 1, column 0, while row 3, column 0 calculates `$2018`, just beyond the grid. A column walk using column 4 would eventually read beyond the grid as well. ## Instructions in this Lecture - M68K: [`lea`](https://asm-editor.specy.app/documentation/m68k/instruction/lea.md), [`move`](https://asm-editor.specy.app/documentation/m68k/instruction/move.md), [`mulu`](https://asm-editor.specy.app/documentation/m68k/instruction/mulu.md), [`add`](https://asm-editor.specy.app/documentation/m68k/instruction/add.md), [`moveq`](https://asm-editor.specy.app/documentation/m68k/instruction/moveq.md), [`clr`](https://asm-editor.specy.app/documentation/m68k/instruction/clr.md), [`dbra`](https://asm-editor.specy.app/documentation/m68k/instruction/dbra.md) Previous: [Binary search](https://asm-editor.specy.app/learn/courses/m68k/examples/binary-search.md) Next: [Print a string](https://asm-editor.specy.app/learn/courses/m68k/examples/hello-world.md)