That's simply not true.�� In fact the amount known of the VM in both the bytecode debugger and the AST debugger is the same; both use VM primitives that aren't specified at the image level.

Both the AST interpreter and the bytecode interpreter include the interpreter in Smalltalk in the image.�� Look at Context and InstructionStream to find the Smalktalk definitions of both the interpreter routines and the implementations of the bytecodes.

So my question is why our debuggers are not based on AST interpreter? Why it is bad idea? (if it is bad).

One area where you'll find the AST debugger ��worse is in performance.�� The bytecode debugger uses perform: to execute at full speed when possible (it does interpret when doing step into or through, but not when doing over).�� In fact this part of the system, because it has to catch no local returns and exceptions, is the most complex, but it's not because of VM details, it's because of the use of perform: to run code being debugged to run at full speed.

Not to mention, if it isn't broke don't fix it.

But seriously, the system is based on bytecode, and that bytecode is pretty straightforward and easy to learn.�� Why not put in the effort of learning it?

Criticizing something in ignorance is not wise IMO.

InstructionStream >>interpretNextV3PlusClosureInstructionFor: client��
"Send to the argument, client, a message that specifies the type of the��
next instruction."
| byte type offset method |
method := self method. ��
byte := method at: pc.
type := byte // 16. ��
offset := byte \\ 16. ����
pc := pc+1.
"We do an inline binary search on each of the possible 16 values of type:
The old, cleaner but slowe code is retained as a comment below"
type < 8
ifTrue: [type < 4
ifTrue: [type < 2
ifTrue: [type < 1
ifTrue: ["type = 0"
^ client pushReceiverVariable: offset]
ifFalse: ["type = 1"
^ client pushTemporaryVariable: offset]]
ifFalse: [type < 3
ifTrue: ["type = 2"
^ client
pushConstant: (method literalAt: offset + 1)]
ifFalse: ["type = 3"
^ client
pushConstant: (method literalAt: offset + 17)]]]
ifFalse: [type < 6
ifTrue: [type < 5
ifTrue: ["type = 4"
^ client
pushLiteralVariable: (method literalAt: offset + 1)]
ifFalse: ["type = 5"
^ client
pushLiteralVariable: (method literalAt: offset + 17)]]
ifFalse: [type < 7
ifTrue: ["type = 6"
offset < 8
ifTrue: [^ client popIntoReceiverVariable: offset]
ifFalse: [^ client popIntoTemporaryVariable: offset - 8]]
ifFalse: ["type = 7"
offset = 0
ifTrue: [^ client pushReceiver].
offset < 8
ifTrue: [^ client
pushConstant: (SpecialConstants at: offset)].
offset = 8
ifTrue: [^ client methodReturnReceiver].
offset < 12
ifTrue: [^ client
methodReturnConstant: (SpecialConstants at: offset - 8)].
offset = 12
ifTrue: [^ client methodReturnTop].
offset = 13
ifTrue: [^ client blockReturnTop].
offset > 13
ifTrue: [^ self unusedBytecode: client at: pc - 1 ]]]]]
ifFalse: [type < 12
ifTrue: [type < 10
ifTrue: [type < 9
ifTrue: ["type = 8"
^ self
interpretV3PlusClosureExtension: offset��
in: method��
for: client]
ifFalse: ["type = 9 (short jumps)"
offset < 8
ifTrue: [^ client jump: offset + 1].
^ client jump: offset - 8 + 1 if: false]]
ifFalse: [type < 11
ifTrue: ["type = 10 (long jumps)"
byte := method at: pc.
pc := pc + 1.
offset < 8
ifTrue: [^ client jump: offset - 4 * 256 + byte].
^ client jump: (offset bitAnd: 3)
* 256 + byte if: offset < 12]
ifFalse: ["type = 11"
^ client
send: (Smalltalk specialSelectorAt: offset + 1)
super: false
numArgs: (Smalltalk specialNargsAt: offset + 1)]]]
ifFalse: [type = 12
ifTrue: [^ client
send: (Smalltalk specialSelectorAt: offset + 17)
super: false
numArgs: (Smalltalk specialNargsAt: offset + 17)]
ifFalse: ["type = 13, 14 or 15"
^ client
send: (method literalAt: offset + 1)
super: false
numArgs: type - 13]]].

" �� ��old code��
type=0 ifTrue: [^client pushReceiverVariable: offset].
type=1 ifTrue: [^client pushTemporaryVariable: offset].
type=2 ifTrue: [^client pushConstant: (method literalAt: offset+1)].
type=3 ifTrue: [^client pushConstant: (method literalAt: offset+17)].
type=4 ifTrue: [^client pushLiteralVariable: (method literalAt: offset+1)].
type=5 ifTrue: [^client pushLiteralVariable: (method literalAt: offset+17)].
type=6��
ifTrue: [offset<8
ifTrue: [^client popIntoReceiverVariable: offset]
ifFalse: [^client popIntoTemporaryVariable: offset-8]].
type=7
ifTrue: [offset=0 ifTrue: [^client pushReceiver].
offset<8 ifTrue: [^client pushConstant: (SpecialConstants at: offset)].
offset=8 ifTrue: [^client methodReturnReceiver].
offset<12 ifTrue: [^client methodReturnConstant:��
(SpecialConstants at: offset-8)].
offset=12 ifTrue: [^client methodReturnTop].
offset=13 ifTrue: [^client blockReturnTop].
offset>13 ifTrue: [^self error: 'unusedBytecode']].
type=8 ifTrue: [^self interpretExtension: offset in: method for: client].
type=9
ifTrue: ��short jumps
[offset<8 ifTrue: [^client jump: offset+1].
^client jump: offset-8+1 if: false].
type=10��
ifTrue: ��long jumps
[byte:= method at: pc. ��pc:= pc+1.
offset<8 ifTrue: [^client jump: offset-4*256 + byte].
^client jump: (offset bitAnd: 3)*256 + byte if: offset<12].
type=11��
ifTrue:��
[^client��
send: (Smalltalk specialSelectorAt: offset+1)��
super: false
numArgs: (Smalltalk specialNargsAt: offset+1)].
type=12��
ifTrue:��
[^client��
send: (Smalltalk specialSelectorAt: offset+17)��
super: false
numArgs: (Smalltalk specialNargsAt: offset+17)].
type>12
ifTrue:��
[^client send: (method literalAt: offset+1)��
super: false
numArgs: type-13]"��