Analysis of information sources in references of the Wikipedia article "Halt and Catch Fire (computing)" in English language version.
The mnemonics are, of course, assigned by me.
To add insult to injury, we discovered that we had an illegal HACOF, an instruction that our customers found on the MC6800. It was an unused opcode-an illegal instruction. When executed inadvertently, the program counter would increment indefinitely. The problem, which was caused by incomplete opcode decoding, was a nuisance because Reset was the only means of terminating the instruction. ... During the design process, we figured out how to eliminate the HACOF instruction. About that time, the product engineers came to us with an idea. They said, 'You know what we'd really like? Some way to quickly test the RAM. If we could somehow point the program counter at the first RAM address and then just increment through the RAM, we could test it a lot faster.' Since the HACOF 'instruction' did precisely that—and we really didn't want to invest the effort needed to remove it—we replied, 'Have we got a deal for you!' HACOF thus became the first intentional built-in self-test feature on a Motorola microprocessor.
There are a lot of details that the engineer designing the "microprogram" must keep in mind, otherwise short circuits can destroy the hardware. The Z1 with its mechanical design was still more sensitive in this respect than the Z3. Even after it was completed, there were sequences of instructions that the programmer had to avoid in order not to damage the hardware. One of those sequences was inadvertently tried at the Berlin Museum of Technology and Transportation, which led to slight damaging of the reconstructed Z1 in 1994.(12 pages)
There are a lot of details that the engineer designing the "microprogram" must keep in mind, otherwise short circuits can destroy the hardware. The Z1 with its mechanical design was still more sensitive in this respect than the Z3. Even after it was completed, there were sequences of instructions that the programmer had to avoid in order not to damage the hardware. One of those sequences was inadvertently tried at the Berlin Museum of Technology and Transportation, which led to slight damaging of the reconstructed Z1 in 1994.(12 pages)
To add insult to injury, we discovered that we had an illegal HACOF, an instruction that our customers found on the MC6800. It was an unused opcode-an illegal instruction. When executed inadvertently, the program counter would increment indefinitely. The problem, which was caused by incomplete opcode decoding, was a nuisance because Reset was the only means of terminating the instruction. ... During the design process, we figured out how to eliminate the HACOF instruction. About that time, the product engineers came to us with an idea. They said, 'You know what we'd really like? Some way to quickly test the RAM. If we could somehow point the program counter at the first RAM address and then just increment through the RAM, we could test it a lot faster.' Since the HACOF 'instruction' did precisely that—and we really didn't want to invest the effort needed to remove it—we replied, 'Have we got a deal for you!' HACOF thus became the first intentional built-in self-test feature on a Motorola microprocessor.
There are a lot of details that the engineer designing the "microprogram" must keep in mind, otherwise short circuits can destroy the hardware. The Z1 with its mechanical design was still more sensitive in this respect than the Z3. Even after it was completed, there were sequences of instructions that the programmer had to avoid in order not to damage the hardware. One of those sequences was inadvertently tried at the Berlin Museum of Technology and Transportation, which led to slight damaging of the reconstructed Z1 in 1994.(12 pages)
