read all other bits of mpu-401 status register as 1HEADv0.3.3master

this improves compatibility with windows nt 3/4's builtin soundblaster
driver

1 files changed, 4 insertions, 1 deletions

diff --git a/Mpu401.cpp b/Mpu401.cpp
index 450aedd..eadd1a2 100644
--- a/Mpu401.cpp
+++ b/Mpu401.cpp
@@ -301,7 +301,10 @@ static uint8_t mpuReadStatus(PPDMDEVINS pDevIns)
          0 - Data is available for reading
          1 - No data is available for reading */
 
-    uint8_t status = 0;
+    /* All other bits are undefined, but software such as
+     * NT's sndblst.sys expects them to be read as 1. */
+
+    uint8_t status = 0xFFU & ~(RT_BIT(6) | RT_BIT(7));
 
     Log7Func(("tx buf=%zu/%zu rx buf=%zu/%zu\n",
               RTCircBufUsed(pThis->pTxBuf), RTCircBufFree(pThis->pTxBuf),