From 710c6c6f2f08f7d938f552a0272d2424cf9fdc68 Mon Sep 17 00:00:00 2001
From: Greg King <gregdk@users.sf.net>
Date: Wed, 13 Jan 2021 03:20:19 -0500
Subject: [PATCH] Fixed cbm_k_readst() to work around a VIC-20 Kernal bug.

It properly returns the RS-232 device's status.
---
 asminc/vic20.inc        |  9 +++++----
 libsrc/cbm/c_readst.s   |  8 ++++----
 libsrc/vic20/c_readst.s | 28 ++++++++++++++++++++++++++++
 3 files changed, 37 insertions(+), 8 deletions(-)
 create mode 100644 libsrc/vic20/c_readst.s

diff --git a/asminc/vic20.inc b/asminc/vic20.inc
index fa9fdfa8d..91da083f1 100644
--- a/asminc/vic20.inc
+++ b/asminc/vic20.inc
@@ -1,8 +1,7 @@
 ;
-; Vic20 generic definitions. Stolen mostly from c64.inc - Steve Schmidtke
+; VIC-20 generic definitions. Stolen mostly from c64.inc -- Steve Schmidtke
 ;
 
-
 ; ---------------------------------------------------------------------------
 ; Zero page, Commodore stuff
 
@@ -12,8 +11,8 @@ TXTPTR          := $7A          ; Pointer into BASIC source code
 STATUS          := $90          ; Kernal I/O completion status
 TIME            := $A0          ; 60HZ clock
 FNAM_LEN        := $B7          ; Length of filename
-SECADR          := $B9          ; Secondary address
-DEVNUM          := $BA          ; Device number
+SECADR          := $B9          ; Second address
+DEVNUM          := $BA          ; Device number (first address)
 FNAM            := $BB          ; Pointer to filename
 KEY_COUNT       := $C6          ; Number of keys in input buffer
 RVS             := $C7          ; Reverse flag
@@ -36,6 +35,8 @@ KBDREPEAT       := $28a
 KBDREPEATRATE   := $28b
 KBDREPEATDELAY  := $28c
 
+RSSTAT          := $297         ; RS-232 device driver status
+
 ; ---------------------------------------------------------------------------
 ; Screen size
 
diff --git a/libsrc/cbm/c_readst.s b/libsrc/cbm/c_readst.s
index 85211dd2f..bcaacbbae 100644
--- a/libsrc/cbm/c_readst.s
+++ b/libsrc/cbm/c_readst.s
@@ -1,5 +1,6 @@
 ;
-; Ullrich von Bassewitz, 03.06.1999
+; 1999-06-03, Ullrich von Bassewitz
+; 2021-01-12, Greg King
 ;
 ; unsigned char cbm_k_readst (void);
 ;
@@ -10,6 +11,5 @@
 
 
 _cbm_k_readst:
-        jsr     READST
-        ldx     #0              ; Clear high byte
-        rts
+        ldx     #>$0000
+        jmp     READST
diff --git a/libsrc/vic20/c_readst.s b/libsrc/vic20/c_readst.s
new file mode 100644
index 000000000..2cb11bb21
--- /dev/null
+++ b/libsrc/vic20/c_readst.s
@@ -0,0 +1,28 @@
+;
+; 1999-06-03, Ullrich von Bassewitz
+; 2021-01-12, Greg King
+;
+; unsigned char cbm_k_readst (void);
+;
+; This version works around a bug in VIC-20 Kernal's READST function.
+;
+
+        .include        "vic20.inc"
+        .include        "../cbm/cbm.inc"
+
+        .export         _cbm_k_readst
+
+
+_cbm_k_readst:
+        ldx     #>$0000
+        lda     DEVNUM
+        cmp     #CBMDEV_RS232
+        beq     @L1
+
+        jmp     READST
+
+; Work-around:  Read the RS-232 status variable directly.
+
+@L1:    lda     RSSTAT
+        stx     RSSTAT          ; reset the status bits
+        rts