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- tools - asm-11/lib/defs_xu.mac: add xt.* and xr.* definitions - oskit/211bsd_rk/211bsd_rk_boot.ecmd: add tt1: mode: definition - tcl/rw11/shell.tcl: update '.h' and '.ha' text output - tcode/cpu_basics.mac: A4.4: change exemptions, e11 now like 11/70 and w11
56 lines
2.2 KiB
Markdown
56 lines
2.2 KiB
Markdown
## Known differences between SimH, 11/70, and w11a
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### SimH: `JSR SP` pushes modified `SP` value
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In the logic of the `JSR` instruction is the given register the
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_linkage register_, meant to be used for parameter passing.
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`JSR` pushes the _linkage register_ to the stack, and the matching `RTS`
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will restore it. That works fine for `R0` to `R5` and for `PC`.
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But in a `JSR SP` that interferes with the implicit use of `SP`
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as the stack pointer, `SP` is saved on the stack which is defined by `SP`.
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The question is whether the original `SP` value is saved, or the `SP`
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value after it has been decremented to hold the address for the write
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to stack. The specification simply says
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```
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down(sp) := sp
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```
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and the question is whether the side effect of the left side happens before
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the right side is evaluated.
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The 11/70 implementation saves the value of the linkage register value in
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an internal register in `JSR.00` before the `SP` is decremented in `JSR.20`
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and write the saved valued to stack in `JSR.30`. See flow 11:
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```
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JSR.00: PCA := DR ; save dst addr in PCA
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SR := GS[SF] if -SF7 ; get modified source field reg to SR
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SR := PCB if SF7 ; get modified source field reg to SR
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JSR.10: DR := GS[6] ; get SP
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BR := SR ; get modified source field reg to BR
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JSR.20 DR,GR[6] := DR-2 ; step SP back for push
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JSR.30 write BR to DR ; write old source field to stack
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JSR.40 GR[SF] := PCB ; old PC goes to src field reg
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PCB := PCA ; dst addr goes tp PC
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```
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Therefore, the 11/70 writes the original `SP` value.
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SimH writes the modified `SP`
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```c
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srcspec = srcspec & 07;
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dst = GeteaW (dstspec);
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SP = (SP - 2) & 0177777;
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WriteW (R[srcspec], SP | dsenable);
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R[srcspec] = PC;
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JMP_PC (dst & 0177777);
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```
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`JSR SP` is never used due to its bizarre behavior. The matching `RTS SP`
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results in a useless `SP` too. Given that, this is considered an
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acceptable deviation from 11/70 behavior.
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The w11 correctly implements the 11/70 behavior.
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This is verified in a [tcode](../tools/tcode/README.md), the test is
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modified when executed on SimH
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(see [cpu_basics](../tools/tcode/cpu_basics.mac) tests A4.4).
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Tested with SimH V3.12-3.
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