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* 0078-001-H102 (G$DR) DRAWING NO. 41285543-001-03 PAGE 4

0151 * 0152 * THE FOUR ROUTINES TABS, SKP, NOP AND ERNA SATISFY ANY 0153 * OUTSTANDING CALLS FOR ANY OF THE 85 INPUT/OUTPUT LIBRARY 0154 * ROUTINES LISTED IN THE 'SUBR' SECTION AT THE BEGINNING OF 0155 * THE CODING BELOW. THE ROUTINES TABS, SKP AND NOP CAUSE THE 0156 * RELEVANT CALLS TO BE IGNORED AT RUN TIME, TOGETHER WITH ANY 0157 * ASSOCIATED PARAMETERS. HOWEVER ALL CALLS ASSOCIATED WITH 0158 * THE ROUTINE ERNA INDICATE AN ERROR CONDITION (SEE NEXT 0159 * PARAGRAPH). 0160 * 0161 * 0162 * ERRORS 0163 * 0164 * ANY CALLS ON INPUT-OUTPUT ROUTINES ASSOCIATED WITH THE DEFAULT 0165 * ROUTINE ERNA CAUSE THE EXTERNAL ROUTINE G$NA (DEVICE-NOT-AVAILABLE 0166 * ERROR ROUTINE) TO BE CALLED. NO RETURN IS POSSIBLE AFTER THIS 0167 * CALL. G$NA MUST BE PROVIDED AS AN ENTRY (SUBR) TO THE INPUT- 0168 * OUTPUT SUPERVISOR, WHICH MAY, FOR EXAMPLE, PRINT AN ERROR MESSAGE. 0169 * 0170 * 0171 * METHOD 0172 * 0173 * THE FIRST TIME G$DR (GENERIC DUMMY ROUTINE) IS CALLED, THE 0174 * APPROPRIATE ADDRESSING MODE - DXA OR EXA - IS DETERMINED, 0175 * ACCORDING TO THE MODE IN WHICH THE PROGRAM WAS DESECTORED ON 0176 * LOADING. THE ACTUAL CORE SIZE AVAILABLE IN THIS ADDRESSING 0177 * MODE IS DETERMINED, THE RESULTS ARE STORED FOR FUTURE 0178 * REFERENCE AND THE LOCATIONS CONTAINING THE ALGORITHM ARE 0179 * RELEASED TO FORM PART OF THE FREE CORE AREA. 0180 * 0181 * THE METHOD USED IS BASED ON THE FACT THAT WHEN ONE ATTEMPTS TO 0182 * WRITE A WORD INTO A LOCATION ONE ABOVE THE MAXIMUM CORE SIZE, 0183 * THE WORD IS EITHER LOST OR WRITTEN INTO LOCATION ZERO, 0184 * DEPENDING UPON THE ACTUAL CORE SIZE, WHEN ONE ATTEMPTS TO READ 0185 * THIS WORD BACK IN THE FORMER CASE A ZERO IS READ AND IN THE 0186 * LATTER CASE THE CONTENT OF LOCATION ZERO (THE X-REGISTER) IS 0187 * READ - THIS OF COURSE CAN BE CLEARED BEFOREHAND. IF THE PROGRAM 0188 * IS RUNNING IN DXA MODE IN A MACHINE WITH A CORE SIZE GREATER THAN 0189 * 16K, THE CORE SIZE WILL STILL APPEAR TO BE 16K - WHICH IS CORRECT 0190 * FOR THE MODE OF USE 0191 * 0192 * G$DR CONTAINS ONE INSTRUCTION WHICH MUST BE DESECTORED, AND TO 0193 * ENABLE THE USER TO AVOID MAKING USE OF THE NORMAL BASE SECTOR 0194 * FOR CERTAIN SPECIALISED APPLICATIONS, THE BASE SECTOR IS RESET 0195 * BEFORE THIS INSTRUCTION. 0196 * 0197 * 0198 ************************************************************************ 0199 * 0200 *