(r3) CallingCFunction < MecrispCube

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---+ Calling C Functions from Forth and Vice Versa

---++ Register Usage
Procedure Call Standard for the ARM Architecture [[https://developer.arm.com/documentation/ihi0042/j][AAPCS]]

https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/on-the-aapcs-with-an-application-to-efficient-parameter-passing

| *Register* | *AAPCS Purpose*                                         | *MECRISP Forth Purpose*  | *ISR Stacking*  |
| xPSR       | Processor Status Register, Flags                        | Flags                    | 1               | 
| r0         | Argument, result, scratch register 1                    | Scratch                  | 2               |
| r1         | Argument, result, scratch register 2                    | Scratch                  | 3               |
| r2         | Argument, scratch register 3                            | Scratch                  | 4               |
| r3         | Argument, scratch register 4                            | Scratch                  | 5               |
| r4         | Variable register 1                                     | DO Index                 |                 |
| r5         | Variable register 2                                     | DO Limit                 |                 |
| r6         | Variable register 3                                     | Top Of Stack TOS         |                 |
| r7         | Variable register 4                                     | Data stack pointer       |                 |
| r8         | Variable register 5                                     | unused                   |                 |
| r9         | Platform register (usage defined by platform in use)    | unused                   |                 |
| r10        | Variable register 7                                     | unused                   |                 |
| r11        | Variable register 8                                     | unused                   |                 |
| r12        | Intra-procedure-call scratch register                   | unused                   | 6               |
| r13        | Stack pointer (SP)                                      | Stack pointer (SP)       |                 |
| r14        | Link register (LR)                                      | Link register (LR)       | 7               |
| r15        | Program counter (PC)                                    | Program counter (PC)     | 0               |

---++ VFP Register Usage
The VFP-v2 co-processor has 32 single-precision registers, s0-s31, which may also be accessed as 16
double-precision registers, d0-d15 (with d0 overlapping s0, s1; d1 overlapping s2, s3; etc).

---++ Calling C Functions from Forth
---++ Calling Forth Words from C Functions
Stack Pointer and Top Of Stack

-- %USERSIG{PeterSchmid - 2020-07-13}%

---++ Comments

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Topic revision: r3 - 2020-07-17 - PeterSchmid