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cpu_abi [2021/08/29 20:28] someone2639 [Functions With More Than Four Arguments] made RA location clearer |
cpu_abi [2021/10/16 17:31] (current) someone2639 [Functions With Floating/Decimal Arguments] asm corrections |
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| ==== Functions With More Than Four Arguments ==== | ==== Functions With More Than Four Arguments ==== | ||
| - | The ABI specifies four argument registers, so the natural thing to wonder is where a to put a fifth argument, sixth argument, and so on. The answer is to put them on the Stack. The general layout of the stack is as shown: | + | The ABI specifies four argument registers, so the natural thing to wonder is where a to put a fifth argument, sixth argument, and so on. The answer is to put them on the Stack. The general layout of a stack frame (the view of the stack from any arbitrary function) is as shown: |
| == General Stack Usage == | == General Stack Usage == | ||
| Line 89: | Line 89: | ||
| The ''addiu'' instruction determines how much gets allocated to the stack, and the return address is placed at the top of this region. | The ''addiu'' instruction determines how much gets allocated to the stack, and the return address is placed at the top of this region. | ||
| + | ==== Functions With Floating/Decimal Arguments ==== | ||
| + | |||
| + | When a function takes floating point arguments, the ABI assumes you're using float registers more often and gives each of them their own purpose so you dont have to use general purpose registers too much. | ||
| + | |||
| + | ^ C Function ^ Equivalent ASM ^ | ||
| + | | <code c> | ||
| + | float three_input_adder(float a, float b, float c) { | ||
| + | return a + b + c; | ||
| + | } | ||
| + | void myFunc(void) { | ||
| + | float x = three_input_adder(1.0f, 3.0f, 4.0f); | ||
| + | } | ||
| + | </code> | <code> | ||
| + | glabel three_input_adder | ||
| + | lwc1 f4, 0x10(sp) | ||
| + | add.s f0, f12, f14 | ||
| + | add.s f0, f0, f4 ; return value gets stored in f0 | ||
| + | jr ra | ||
| + | nop | ||
| + | glabel myFunc | ||
| + | addiu sp, -0x18 | ||
| + | sw ra, 0x14(sp) | ||
| + | | ||
| + | li.s f12, 1.0 ; f12 holds the first argument | ||
| + | li.s f14, 3.0 ; f14 holds the second argument | ||
| + | li.s f4, 4.0 | ||
| + | swc1 f4, 0x10(sp) ; subsequent float arguments go on the stack | ||
| + | jal three_input_adder | ||
| + | nop | ||
| + | | ||
| + | mfc1 t0, f0 ; f0 has the result | ||
| + | | ||
| + | lw ra, 0x14(sp) | ||
| + | jr ra | ||
| + | addiu sp, 0x18 | ||
| + | </code> | | ||
cpu_abi.1630268925.txt.gz ยท Last modified: 2021/08/29 20:28 by someone2639