I had worked with the following (30) system fields:
1) SY-DBSYS - Central Database
2) SY-HOST - Server
3) SY-OPSYS - Operating System
4) SY-SAPRL - SAP Release
5) SY-SYSID - System Name
6) SY-LANGU - User Logon Language
7) SY-MANDT - Client
8) SY-UNAME - Logon User Name
9) SY-DATLO - Local Date
10) SY-DATUM - Server Date
11) SY-TIMLO - Local Time
12) SY-UZEIT - Server Time
13) SY-DYNNR - Screen Number
14) SY-REPID - Current ABAP program
15) SY-TCODE - Transaction Code
16) SY-ULINE - Horizontal Line
17) SY-VLINE - Vertical Line
18) SY-INDEX - Number of current loop Pass
19) SY-TABIX - Current line of internal table
20) SY-DBCNT - Number of table entries processed
21) SY-SUBRC - Return Code
22) SY-UCOMM - Function Code
23) SY-LINCT - Page Length of list
24) SY-LINNO - Current Line
25) SY-PAGNO - Current Page Number
26) SY-LSIND - Index of List
27) SY-MSGID - Message Class
28) SY-MSGNO - Message Number
29) SY-MSGTY - Message Type
30) SY-SPONO - Spool number during printing