[PATCH v4 1/6] target/m68k: Add dummy CAAR register

[54weasels <54weasels@gmail.com>]

High level description:
The Sun-3 ROM expects the Cache Control Register (CAAR) to exist on the 68020. Reading/writing this register currently aborts QEMU. This patch provides a dummy implementation that simply ignores writes and returns 0 on reads, allowing the ROM to continue booting.

Impact on existing functionality:
No impact on existing targets. ColdFire and Mac targets do not access CAAR in a way that currently aborts, and providing a safe dummy implementation ensures robustness across M68k boards.

Context: This patch was originally submitted as part of the monolithic Sun-3 Machine Emulation series (https://patchew.org/QEMU/20260503015756.99176-1-54weasels@gmail.com/) and has been split into atomic components.
---
 target/m68k/helper.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/target/m68k/helper.c b/target/m68k/helper.c
index 5f91d206f5..68f523ea84 100644
--- a/target/m68k/helper.c
+++ b/target/m68k/helper.c
@@ -281,8 +281,10 @@ void HELPER(m68k_movec_to)(CPUM68KState *env, uint32_t reg, uint32_t val)
             return;
         }
         break;
-    /* Unimplemented Registers */
+    /* Dummy implementation for CAAR */
     case M68K_CR_CAAR:
+        return;
+    /* Unimplemented Registers */
     case M68K_CR_PCR:
     case M68K_CR_BUSCR:
         cpu_abort(env_cpu(env),
@@ -385,8 +387,10 @@ uint32_t HELPER(m68k_movec_from)(CPUM68KState *env, uint32_t reg)
             return env->mmu.ttr[M68K_DTTR1];
         }
         break;
-    /* Unimplemented Registers */
+    /* Dummy implementation for CAAR */
     case M68K_CR_CAAR:
+        return 0;
+    /* Unimplemented Registers */
     case M68K_CR_PCR:
     case M68K_CR_BUSCR:
         cpu_abort(env_cpu(env), "Unimplemented control register read 0x%x\n",
-- 
2.50.1 (Apple Git-155)