amiga: replace magic numbers in startup.S with named constants

platform/amiga/startup.S

@@ -11,50 +11,78 @@
 	.globl	_start
 	.globl	_entrypoint
 
+/* The NDK ships its asm headers (exec/tasks.i, lvo/exec_lib.i) in
+ * Amiga assembler syntax (IFND, EQU, ...), which GNU as does not
+ * understand. We define the few constants we need here.
+ */
+
+/* Exec library vector offsets (from NDK lvo/exec_lib.i) */
+#define _LVOFindTask	-294
+#define _LVOAllocMem	-198
+#define _LVOFreeMem	-210
+#define _LVOStackSwap	-732
+
+/* struct Task offsets (from NDK exec/tasks.i) */
+#define TC_SPLOWER	58
+#define TC_SPUPPER	62
+
+/* struct StackSwapStruct offsets (from NDK exec/tasks.i) */
+#define STK_LOWER	0
+#define STK_UPPER	4
+#define STK_POINTER	8
+#define STK_SIZEOF	12
+
+/* AllocMem flags (from NDK exec/memory.i) */
+#define MEMF_PUBLIC	0x00000001
+#define MEMF_CLEAR	0x00010000
+
+#define MIN_STACK	4096
+#define SWAP_STACK	8192
+
 _start:
 _entrypoint:
 	move.l	4.w,a6			/* SysBase */
 	sub.l	a1,a1
-	jsr	-0x126(a6)		/* FindTask(NULL) */
+	jsr	_LVOFindTask(a6)	/* FindTask(NULL) */
 	move.l	d0,a4			/* a4 = our task */
 
-	/* Check stack — we need at least 4096 bytes */
-	move.l	62(a4),d0		/* tc_SPUpper */
-	sub.l	58(a4),d0		/* tc_SPLower */
-	cmp.l	#4096,d0
+	/* Check stack — swap to a larger one if too small */
+	move.l	TC_SPUPPER(a4),d0
+	sub.l	TC_SPLOWER(a4),d0
+	cmp.l	#MIN_STACK,d0
 	bge.s	.stack_ok
 
 	/* Stack too small — allocate a new one */
-	move.l	#8192,d0
-	move.l	#0x10001,d1		/* MEMF_PUBLIC|MEMF_CLEAR */
-	jsr	-0xc6(a6)		/* AllocMem */
+	move.l	#SWAP_STACK,d0
+	move.l	#MEMF_PUBLIC+MEMF_CLEAR,d1
+	jsr	_LVOAllocMem(a6)
 	tst.l	d0
 	beq.s	.fail
 	move.l	d0,a5			/* a5 = stack base */
 
 	/* Set up StackSwap structure on current stack */
-	sub.l	#12,sp
-	move.l	a5,(sp)			/* stk_Lower */
-	lea	8192(a5),a0
-	move.l	a0,4(sp)		/* stk_Upper */
-	move.l	a0,8(sp)		/* stk_Pointer (top of stack) */
-	move.l	sp,a2			/* save StackSwap struct addr */
+	sub.l	#STK_SIZEOF,sp
+	move.l	a5,STK_LOWER(sp)
+	lea	SWAP_STACK(a5),a0
+	move.l	a0,STK_UPPER(sp)
+	move.l	a0,STK_POINTER(sp)	/* initial SP = top of new stack */
+	move.l	sp,a2			/* save struct addr (callee-saved) */
 	move.l	a2,a0
-	jsr	-0x2dc(a6)		/* StackSwap */
+	jsr	_LVOStackSwap(a6)
 
 	jsr	_handler_main		/* C entry point */
 
-	/* Swap stack back to original */
+	/* Swap back to original stack (struct still on old stack via a2) */
 	move.l	4.w,a6
-	move.l	a2,a0			/* StackSwap struct on old stack */
-	jsr	-0x2dc(a6)		/* StackSwap — restore original stack */
+	move.l	a2,a0
+	jsr	_LVOStackSwap(a6)
 
 	/* Free the allocated stack */
-	move.l	a5,a1			/* stack memory base */
-	move.l	#8192,d0
-	jsr	-0xd2(a6)		/* FreeMem */
+	move.l	a5,a1
+	move.l	#SWAP_STACK,d0
+	jsr	_LVOFreeMem(a6)
 
-	lea	12(sp),sp		/* remove StackSwap struct */
+	lea	STK_SIZEOF(sp),sp	/* pop StackSwap struct */
 	bra.s	.done
 
 .stack_ok: