[vcf-midatlantic] Sep 28 2019 workshop followup
DBC1964 at cox.net
Sat Oct 19 08:38:48 EDT 2019
Our 1219 computer has some similar instructions: SADD stores the lower
12 bits of the lower accumulator in the address field of an instruction
in memory. There are instructions to save the states of the index
control register(ICR) and the SR register(page) usually into an Enter
ICR instruction or a Enter SR instruction(enter is UNIVAC for load) to
save/restore processor state on entering / exiting a sub-routine. The
Return Jump instruction stored the program counter +1 into the first
address of a subroutine and jumped to the following address. Exit is by
an Indirect Jump to the same address.
On 10/17/2019 5:48 AM, Jeffrey Jonas via vcf-midatlantic wrote:
> For example, there are 2 instructions
> for modifying the address field on an instuction.
> One saved the program counter plus 1 in the address field of an instruction.
> That's how it did subroutines: store the return address in the
> jump/branch instruction
> at the END of the subroutine, then jump to the subroutine start.
> Another saved the accumulator to the address field of an instruction.
> That was probably for address calculations in lieu of index registers.
> It's essentially a "move under mask"
> since the address field was NOT the entire word.
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