Add documentation about C/ASM calling conventions.

README.md

@@ -5,3 +5,44 @@ Demo for the Sanco 800x.
 The purpose of this demo is mainly to thinker with the hardware of the mentioned computer, since we are reverse-engineering it, and even writing an emulator.
 
 This repository is part of the [CEDA project](https://github.com/GLGPrograms/ceda-home) by [Retrofficina GLG Programs](https://retrofficina.glgprograms.it/).
+
+## C/ASM calling conventions
+Because the folks at z88dk were not able to put this plain and simple table
+in their messy documentation :smile:
+
+### Prologue
+```
+    push    ix
+    ld      ix,$0000
+    add     ix,sp
+```
+
+### Parameters stack
+```
+    uint8_t
+        ix+$04
+
+    uint8_t, uint8_t
+        ix+$04, ix+$05
+```
+
+### Access call parameters
+```
+    ld      a,(ix+$04)
+    ld      a,(ix+$05)
+```
+
+### Return value
+```
+    uint32_t    de (msb), hl (lsb)
+    uint16_t    hl
+    uint8_t     l
+    void*       hl
+```
+
+### Epilogue
+```
+    pop     ix
+    ret
+```
+

src/main.c

@@ -12,3 +12,19 @@ int main(void) {
 
     return 0;
 }
+
+uint8_t ret_u8(void) {
+    return 0x01;
+}
+
+uint16_t ret_u16(void) {
+    return 0x0123;
+}
+
+uint32_t ret_u32(void) {
+    return 0x01234567;
+}
+
+void *ret_ptr(void) {
+    return (void *)0x1234;
+}