Similarly, the original machines'JUMP instructions landed at a location " one before " the actual address, for reasons similar to the odd index behavior, but these proved useful only in theory and quite annoying in practice, and were similarly modified.
32.
:Jump instructions are kind of in a class by themselves in that they refer to a memory location, but do not actually access that memory location ( the next instruction after a completed jump will be retrieved from that location ).
33.
When the OS Vertical Blank Interrupt is enabled, direct updates to the ANTIC DLIST registers by the CPU or the ANTIC Jump instructions will be overwritten by the OS during the next Vertical Blank by the values in the shadow registers.
34.
:By the time the Scelbi-H was sold to the public, they did have cassette tape loaders and ROM memory-so probably the PDP-11 approach of keying in an initial jump instruction and executing it from the front panel switches was the startup approach.
35.
For example, in such processors a " compare " instruction evaluates two values and sets or clears bits in the flags register to indicate which one is greater or whether they are equal; one of these flags could then be used by a later jump instruction to determine program flow.
36.
Upon starting, an IBM-compatible personal computer's x86 CPU executes, in real mode, the instruction located at reset vector ( the physical memory address FFFF0h on 16-bit x86 processors and FFFFFFF0h on 32-bit and 64-bit x86 processors ), usually pointing to the BIOS entry point inside the ROM . This memory location typically contains a jump instruction that transfers execution to the location of the BIOS start-up program.
