[CBM] Add chkout C routine (#244)

Author: mysterymath

mos-platform/commodore/CMakeLists.txt

@@ -27,6 +27,7 @@ add_platform_library(commodore-c
   cbm_k_basin.c
   cbm_k_bsout.c
   cbm_k_chkin.s
+  cbm_k_chkout.s
   cbm_k_chrin.c
   cbm_k_chrout.c
   cbm_k_cint.c

mos-platform/commodore/cbm.h

@@ -209,6 +209,7 @@ unsigned char cbm_k_acptr (void);
 unsigned char cbm_k_basin (void);
 void cbm_k_bsout (unsigned char C);
 unsigned char cbm_k_chkin (unsigned char FN) __attribute__((leaf));
+unsigned char cbm_k_chkout (unsigned char FN) __attribute__((leaf));
 unsigned char cbm_k_chrin (void);
 void cbm_k_chrout (unsigned char C);
 void cbm_k_cint (void);

mos-platform/commodore/cbm_k_chkout.s

@@ -0,0 +1,13 @@
+.text
+
+;
+; unsigned char cbm_k_chkout (unsigned char FN);
+;
+.global cbm_k_chkout
+cbm_k_chkout:
+	tax
+	jsr __CHKOUT
+	bcs some_error
+	lda #0 ; open was successful if .C = clear
+some_error:
+	rts