Our discussion of SIC/XE Format 4 instructions specified that the 20-bit address field should contain the...

 

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Our discussion of SIC/XE Format 4 instructions specified that the 20-bit "address" field should contain the actual target address, and that addressing mode bits b and p should be set to 0. (That is, the in- struction should contain a direct address-it should not use base rel- ative or program-counter relative addressing.) However, it would be possible to use program-counter relative ad- dressing with Format 4. In that case, the "address" field would actu- ally contain a displacement, and bit p would be set to 1. For example, the instruction on line 15 in Fig. 2.6 could be assembled as 0006 CLOOP +JSUB RDREC 4B30102C (using program-counter relative addressing with displacement 102C). What would be the advantages (if any) of assembling Format 4 instructions in this way? What would be the disadvantages (if any)? Are there any situations in which it would not be possible to assem- ble a Format 4 instruction using program-counter relative address- ing? Our discussion of SIC/XE Format 4 instructions specified that the 20-bit "address" field should contain the actual target address, and that addressing mode bits b and p should be set to 0. (That is, the in- struction should contain a direct address-it should not use base rel- ative or program-counter relative addressing.) However, it would be possible to use program-counter relative ad- dressing with Format 4. In that case, the "address" field would actu- ally contain a displacement, and bit p would be set to 1. For example, the instruction on line 15 in Fig. 2.6 could be assembled as 0006 CLOOP +JSUB RDREC 4B30102C (using program-counter relative addressing with displacement 102C). What would be the advantages (if any) of assembling Format 4 instructions in this way? What would be the disadvantages (if any)? Are there any situations in which it would not be possible to assem- ble a Format 4 instruction using program-counter relative address- ing?