The canonical boot sequence uses WIM 0,2 in location 5 and an initial load address in location 11 (and in the X register). This is limiting for devices like RAD that do not report an end-of-file, or for paper tape that report EOF too late and cause the WIM to fail. A simple load address in location 11, with bit 9 set to satisfy the BRX, doesn't do any word counting. The BRX always branches back to location 5.
This change allows a true negative word count to be placed in location 11 to work with a WIM XXXXX,2 where XXXXX is one location past the region to be loaded by the boot sequence. XXXXX plus the negative count in location 11 (buf[3]+buf[7]) initially points to the first location to be loaded, and the BRX will not branch when the word count is exhausted (register X=0) allowing the branch to the start address to be executed.
Old allowable boot sequence:
location 2 = WIM 12,2
location 3 = BRX 2
location 4 = LDX 11
location 5 = WIM 0,2
location 6 = SKS 21000
location 7 = BRX 5
location 10 = BRU Transfer Address
location 11 = Load address with bit 9 set to force BRX to branch
New allowable boot sequence:
location 2 = WIM 12,2
location 3 = BRX 2
location 4 = LDX 11
location 5 = WIM [end load address + 1],2
location 6 = SKS 21000
location 7 = BRX 5
location 10 = BRU Transfer Address
location 11 = Negative count to number of words to load
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