BRK instruction in MEGA65 Memory/Machine Inspection interface disassembles to 2 bytes

[pchnl]

• Platform: MEGA65, r6
• Matrix Monitor 0.96.20240224.00.3c10488
• Related tickets: "brk should be two bytes?"

When assembling the BRK instruction, it is assembled to 1 byte ($00) and I believe that to be correct (despite the stack manipulation done by the interrupt handling).

When attempting to step through some code with the MEGA65 Memory/Machine Inspection interface (Mega + Tab) I got confused because it shows the original code (00 EE 20 D0 00) which is:
BRK
INC $D020
BRK

as:
BRK $EE <== looks wrong to me
JSR $00D0

By the way, the MEGA65 monitor (MONITOR) displays:
00 BRK
EE 20 D0 INC $D020
00 BRK

Which I think is correct.

[dansanderson]

Relocating this to mega65-core. (Matrix Mode is not powered by the ROM.) MEGA65/mega65-core#843

[SvOlli]

Two bytes for BRK is correct. Here is little hack to show this. It runs on my Sorbus Computer with a 65CE02 (but it's the same with all other 6502 variants).

 35:1000 r a9    :9:$1000: A9 FF       LDA  #$FF
 34:1001 r ff    :0:
 33:1002 r 8d    :9:$1002: 8D FE FF    STA  $FFFE
 32:1003 r fe    :0:
 31:1004 r ff    :0:
 30:fffe w ff    :0:
 29:1005 r a9    :9:$1005: A9 0F       LDA  #$0F
 28:1006 r 0f    :0:
 27:1007 r 8d    :9:$1007: 8D FF FF    STA  $FFFF
 26:1008 r ff    :0:
 25:1009 r ff    :0:
 24:ffff w 0f    :0:
 23:100a r a2    :9:$100A: A2 00       LDX  #$00
 22:100b r 00    :0:
 21:100c r 8e    :9:$100C: 8E 00 DF    STX  $DF00
 20:100d r 00    :0:
 19:100e r df    :0:
 18:df00 w 00    :0:
 17:100f r 00    :9:$100F: 00 EA       BRK  #$EA
 16:1010 r ea    :0:
 15:01fd w 10    :0:
 14:01fc w 11    :0:
 13:01fb w 33    :0:
 12:fffe r ff    :0:
 11:ffff r 0f    :0:
 10:0fff r 40    :9:$0FFF: 40          RTI
  9:1000 r a9    :0:
  8:01fb r 33    :0:
  7:01fc r 11    :0:
  6:01fd r 10    :0:
  5:1011 r ea    :9:$1011: EA          NOP
  4:1012 r 8d    :9:$1012: 8D 01 DF    STA  $DF01
  3:1013 r 01    :0:
  2:1014 r df    :0:
  1:df01 w 0f    :0:

See how at line 17 the BRK is triggered at $100f, but after the RTI the CPU is returning to $1011 at line 5?

This happens with all 6502 variants. So BRK $EE in your example is correct.

It's just that almost every monitor written for the 6502 adjusts the stack afterwards. This is since the 6530-004 (TIM), the companion chip sold with the very first 6502s.

(My kernel uses the BRK for triggering kernel functions, thus eliminating a bigger jump table.)