#516 Show registers and next instruction

[Ark]

(Using Vice 3.10 (GTK3) on Arch Linux.)

There is something about the built-in monitor I find really confusing. The monitor command cpuhistory displays the most recent instructions the CPU has already executed, along with the state before that particular instruction (in the same line) had been executed. Also, the step command displays the state of the CPU after executing the instruction PC is currently pointing at. What is missing here is the state of the CPU right now along with the instruction that is about to be executed. The monitor command registers only tells half the story, as it only shows registers but not the instruction PC is pointing at.

Consider this example (slightly formatted for readability):

(C:$080d) chis 10
.C:e132  48          PHA            A:e1 X:61 Y:0d SP:f8 N.-.....      3132966
.C:e133  A9 46       LDA #$46       A:e1 X:61 Y:0d SP:f7 N.-.....      3132969
.C:e135  48          PHA            A:46 X:61 Y:0d SP:f7 ..-.....      3132971
.C:e136  AD 0F 03    LDA $030F      A:46 X:61 Y:0d SP:f6 ..-.....      3132974
.C:e139  48          PHA            A:00 X:61 Y:0d SP:f6 ..-...Z.      3132978
.C:e13a  AD 0C 03    LDA $030C      A:00 X:61 Y:0d SP:f5 ..-...Z.      3132981
.C:e13d  AE 0D 03    LDX $030D      A:00 X:61 Y:0d SP:f5 ..-...Z.      3132985
.C:e140  AC 0E 03    LDY $030E      A:00 X:00 Y:0d SP:f5 ..-...Z.      3132989
.C:e143  28          PLP            A:00 X:00 Y:00 SP:f5 ..-...Z.      3132993
.C:e144  6C 14 00    JMP ($0014)    A:00 X:00 Y:00 SP:f6 ..-.....      3132997

(C:$080d) z
.C:080e  CA          DEX            - A:00 X:01 Y:00 SP:f6 ..-.....    3133004

Where does X:01 come from? Well, it comes from the instruction at $080d, as we can see afterwards:

(C:$080e) chis 10
.C:e133  A9 46       LDA #$46       A:e1 X:61 Y:0d SP:f7 N.-.....      3132969
.C:e135  48          PHA            A:46 X:61 Y:0d SP:f7 ..-.....      3132971
.C:e136  AD 0F 03    LDA $030F      A:46 X:61 Y:0d SP:f6 ..-.....      3132974
.C:e139  48          PHA            A:00 X:61 Y:0d SP:f6 ..-...Z.      3132978
.C:e13a  AD 0C 03    LDA $030C      A:00 X:61 Y:0d SP:f5 ..-...Z.      3132981
.C:e13d  AE 0D 03    LDX $030D      A:00 X:61 Y:0d SP:f5 ..-...Z.      3132985
.C:e140  AC 0E 03    LDY $030E      A:00 X:00 Y:0d SP:f5 ..-...Z.      3132989
.C:e143  28          PLP            A:00 X:00 Y:00 SP:f5 ..-...Z.      3132993
.C:e144  6C 14 00    JMP ($0014)    A:00 X:00 Y:00 SP:f6 ..-.....      3132997
.C:080d  E8          INX            A:00 X:00 Y:00 SP:f6 ..-.....      3133002

So, looking at the output of the monitor session, we see JMP ($0014) followed by DEX. However, what has actually been executed was JMP ($0014) followed by INX. This is super confusing, at least to me. Sure, I could use disass, but this is far from ergonomic:

• d will print enough to push the current content off the viewport and move the dot address.
• d 080d 080d is no fun to type and will still move the dot address. Also, in order to know that I would have to use 080d, I first need r to tell me the current PC because the dot address might already be off.

Requested change

• Make r show the next instruction that is about to be executed without changing anything, especially without moving the dot address.
• Update the help message for r to reflect that change.

With those changes applied, using r in that scenario might look like this (note the padding at the beginning of the r output for alignment):

(C:$080d) chis 10
.C:e132  48          PHA            A:e1 X:61 Y:0d SP:f8 N.-.....      3132966
.C:e133  A9 46       LDA #$46       A:e1 X:61 Y:0d SP:f7 N.-.....      3132969
.C:e135  48          PHA            A:46 X:61 Y:0d SP:f7 ..-.....      3132971
.C:e136  AD 0F 03    LDA $030F      A:46 X:61 Y:0d SP:f6 ..-.....      3132974
.C:e139  48          PHA            A:00 X:61 Y:0d SP:f6 ..-...Z.      3132978
.C:e13a  AD 0C 03    LDA $030C      A:00 X:61 Y:0d SP:f5 ..-...Z.      3132981
.C:e13d  AE 0D 03    LDX $030D      A:00 X:61 Y:0d SP:f5 ..-...Z.      3132985
.C:e140  AC 0E 03    LDY $030E      A:00 X:00 Y:0d SP:f5 ..-...Z.      3132989
.C:e143  28          PLP            A:00 X:00 Y:00 SP:f5 ..-...Z.      3132993
.C:e144  6C 14 00    JMP ($0014)    A:00 X:00 Y:00 SP:f6 ..-.....      3132997

(C:$080d) r
   ADDR A  X  Y  SP 00 01 NV-BDIZC LIN CYC  STOPWATCH
.; 080d 00 00 00 f6 2f 37 00100000 122 012    3133002
.C:080d  E8          INX

(C:$080d) z
.C:080e  CA          DEX            - A:00 X:01 Y:00 SP:f6 ..-.....    3133004

That is, using r between chis and z will then, finally, tell the whole story.