IBM PC CONVERTIBLE

REGISTERS MODI FI ED : AH AND RETURNED PARAMETER REGISTERS
INVALID FUNCTION REQUESTS (INVALID REGISTER AH VALUES) WILL RETURN AH = 86H
AND THE CARRY FLAG WILL BE SET.
ASSUME OS: OATA, ES: NOTH I NG
001C SYS_SERVICES PROC
FAR
001C 53 PUSH BX
OOlD 8B DC
MOV BX,SP
001F 36: 8B 5F 06
MOV BX,SS: [BX+6]
GET POINTER TO SYSTEM STACK
GET FLAGS FROM SYSTEM STACK
& SAVE IN STK FOR FLG REG LD
RETORE FLAGS
0023 53
0024 90
0025 5B
0026 80 FC 40
0029 72 4E
002B 80 FC 80
002E 73 OA
0030 80 FC 44
0033 n 44
0035 80 EC 40
0038 EB 17
003A
003A 80 FC 91
0030 n 2C
003F 80 EC 7B
0042 80 FC 15
0045 72 05
0047 80 EC 09
004A EB 05
004C
004C
80 FC OC
004F 73 28
0051
0051
1 E
0052 55
0053 50
0054 BD ----
0057 8E DO
0059 2A CO
005B 86 EO
0050 01 EO
005F 8B E8
0061 58
PUSH BX
POPF
POP
CMP
JB
CMP
JAE
BX
AH,40H
INLFUN
AH ,80H
SYS_SV2
RESTORE BX
BELOW SYS SVCS 1 (CSST I/O)?
INVLD PARM OR UNSUPTD CSST
IS SELECT PARM ABOVE 80H?
YES, CK SYS SVCS 2 REQUEST
CHECK FOR SYSTEM SERVICES (AH = 40-44H)
CMP
JA
SUB
JMP
AH,44H
INLFUN
AH ,40H
SHORT SYS_SVC
IN VALID SYS SVCS UP RANGE?
NO, INVALID FUNCTION
YES, SET TABLE OFFSET
GO LINK TO PROPER ROUTINE •..
CHECK FOR SYSTEM SERVICES
(AH = 80-91H)
SYS_SV2 :
CMP
JA
SUB
CMP
JB
SUB
JMP
AH,91H
CHLDESCR
AH,7BH
AH,15H
SYS_SV3
AH,9
SHORT SYS_SVC
IN VlD SYS SVCS 2 UP RANGE?
YES, CK READ DESCRIPTOR ...
MAKE REL TO 4TH ENTRY IN TBl
FUNCTION> = 90H?
NO, GO SYS SERV 3 REQUEST •..
YES, CORRECT THE OFFSET
GO LINK TO THE ROUTINE ..•
CHECK FOR FUNCTIONS 87H-8FH THAT ARE INVALIO
SYS_SV3 :
CMP AH,12
JAE INLFUN
LINK TO PROPER ROUTINE
SYS_SVC:
PUSH
PUSH
PUSH
MOV
MOV
SUB
XCHG
OS
BP
AX
BP,DATA
OS, BP
Al,Al
AH,AL
AX,l
BP ,AX
IS PARM = 87H - 8FH CODE?
YES, INVALID PARAMETER ...
SAVE DS, BP ,AX USE IN SY SERV
SET OS <-- DATA SEGMENT - BP
SET AL TO 0
& FUNCT CODE IN LOW BYTE &
AH=O RET CODE CLR
DOUBLE LINK TABLE OFFSET
GET OFFSET IN BP
RESTORE AX
0062 2E: FF 96 0000 R
SHL
MOV
POP
CALL
POP
POP
JMP
AX
SYS_TABLE[BP]
BP
JUMP TO PROPER ROUTINE
RESTORE BP, OS 0067 50
0068 1F
OS
0069 EB 11 SHORT IMM_RET
SERVICE EXECUTED LEAVE ...
CHECK FOR READ SYSTEM DESCRIPTOR TABLE ADDRESS
006B
006B
006E
0070
0071
0072
0075
oon
80 FC CO
75 09
OE
07
BB 0000 E
2A E4
EB 03
CHLDESCR:
CMP
JNE
PUSH
POP
MOV
SUB
JMP
2-172 ROM BIOS
AH,OCOH IS REQ PARM SYS DESC READ?
INLFUN NO, INVALID FN PARM REQUEST
CS SAVE CURRENT CODE SEGMENT
ES ; SET ES TO POINT TO TABLE
BX,OFFSET SYS_DESCILTABLE ; POINT TO TABLE
AH ,AH CLEAR RETURN CODE
SHORT IMM_RET ; EXIT