[commodore] Implement KERNAL LOAD function as inline assembly (#373)

* Rewrite cbm_k_load w. inline asm

* Add doxygen documentation

* Remove empty line

* Update doxygen documentation

mos-platform/commodore/CMakeLists.txt

@@ -38,7 +38,7 @@ add_platform_library(commodore-c
   cbm_k_getin.c
   cbm_k_iobase.s
   cbm_k_listen.c
-  cbm_k_load.s
+  cbm_k_load.c
   cbm_k_open.s
   cbm_k_readst.c
   cbm_k_save.s

mos-platform/commodore/cbm.h

@@ -231,7 +231,29 @@ void cbm_k_clrch (void);
 unsigned char cbm_k_getin (void);
 unsigned cbm_k_iobase (void) __attribute__((leaf));
 void cbm_k_listen (unsigned char dev);
-void *cbm_k_load(unsigned char flag, void *startaddr) __attribute__((leaf));
+
+/**
+ * @brief CBM KERNAL function to load or verify file
+ *
+ * Should be called after `cbm_k_setlfs()` and `cbm_k_setnam()`.
+ * On Commander X16, the `flag` argument takes the following
+ * values:
+ *
+ * `flag` | Description
+ * ------ | ---------------
+ *   0    | Load at address
+ *   1    | Verify
+ *   2    | Loads into VRAM $00000 + address (cx16)
+ *   3    | Loads into VRAM $10000 + address (cx16)
+ *
+ * On other Commodore targets, a non-zero `flag` value triggers verify.
+ *
+ * @param flag Load or verify
+ * @param load_addr Load address pointer (if not verifying).
+ * @return Address of last byte loaded/verified or KERNAL error code
+ */
+void *cbm_k_load(unsigned char flag, void *load_addr);
+
 unsigned char cbm_k_open (void) __attribute__((leaf));
 unsigned char cbm_k_readst (void);
 unsigned char cbm_k_save(void *startaddr, void *endaddr_plusone) __attribute__((leaf));

mos-platform/commodore/cbm_k_load.c

@@ -0,0 +1,31 @@
+// Copyright 2024 LLVM-MOS Project
+// Licensed under the Apache License, Version 2.0 with LLVM Exceptions.
+// See https://github.com/llvm-mos/llvm-mos-sdk/blob/main/LICENSE for license
+// information.
+
+#include <stdint.h>
+
+// Helper to (dis)assemble a 16-bit integer
+typedef union {
+  uint16_t value;
+  struct {
+    uint8_t lo, hi;
+  };
+} Word;
+
+void *cbm_k_load(const unsigned char flag, void *load_addr) {
+  Word result;
+  const Word addr = {.value = (uint16_t)(load_addr)};
+
+  __attribute__((leaf)) __asm__ volatile(
+      "    jsr __LOAD   \n"
+      "    bcc 1f       \n" // no error if carry clear
+      "    tax          \n" // else get error code from A
+      "    ldy #0       \n"
+      "1:               \n" // no error
+      : "=x"(result.lo), "=y"(result.hi)
+      : "a"(flag), "x"(addr.lo), "y"(addr.hi)
+      : "p");
+
+  return (void *)(result.value);
+}