Implement jump table for syscalls

bdos/8086.asm

@@ -7,24 +7,32 @@ ORG BDOS
 DISKBUF: EQU (BDOS-0x200) ; deblocking
 DISKBPB: EQU (DISKBUF-21) ; BPB of the current driv
 DISKDRV: EQU (DISKBPB-1) ; BYTE denoting drive of current fcb (1=A, ...)
-STACK:   EQU (DISKDRV & 0xFFFE) ; even address, grows down
+
+PROGBX:  EQU (DISKDRV-2)
+
+STACK:   EQU (PROGBX & 0xFFFE) ; even address, grows down
 
 DEFDRV:  EQU 4 ; default drive when opening a FCB (1=A, ...)
 
 SYSCALL:
-  TEST CL, CL
-  JE SETUP
-  CMP CL, 1
-  JE GETC
-  CMP CL, 2
-  JE PUTC
-  CMP CL, 9
-  JE PUTS
-  CMP CL, 10
-  JE GETS
-  CMP CL, 11
-  JE STATUS
-  JMP SYSCALL2
+  MOV [PROGBX], BX
+  XOR BH, BH
+  MOV BL, CL
+  ADD BX, BX
+  ADD BX, FUNCS
+  MOV BX, [BX]
+  PUSH BX
+  MOV BX, [PROGBX]
+SYSRET:
+  RET
+
+FUNCS:
+DW SETUP,GETC,PUTC,SYSRET
+DW SYSRET,SYSRET,SYSRET,SYSRET
+DW SYSRET,PUTS,GETS,STATUS
+DW SYSRET,DISKRST,SETDEFDSK,FCBOPEN
+DW SYSRET,SYSRET,SYSRET,SYSRET
+DW SYSRET,SYSRET,SYSRET,SYSRET
 
 SETUP:
   MOV SP, STACK
@@ -126,16 +134,6 @@ GETS_BS:
 STATUS:
   JMP CONST
 
-SYSCALL2:
-  CMP CL, 13
-  JE DISKRST
-  CMP CL, 14
-  JE SETDEFDSK
-  CMP CL, 15
-  JE FCBOPEN
-RET:
-  RET
-
 DISKRST:
   MOV BYTE [DISKDRV], 0xFF
   RET
@@ -153,6 +151,8 @@ FCBOPEN:
 NODEFDRV:
   CALL LOADBPB
 
+  MOV CX, [DISKBPB+BPB_RDE]
+
   CALL FATSIZE
   ADD DX, WORD [DISKBPB+BPB_RSC]
   CALL SETLSEC
@@ -192,6 +192,7 @@ FATSIZE:
   MUL DL
   XCHG AX, DX
   POP AX
+RET:
   RET
 
 ; IN CL drive num, 1=A, 2=B, 3=C