1. Self-modifying Code
program intentionally modifies code in the program by overwriting machine code
may be used by code designers to obscure branch addresses -> obfuscation - security through obscurity
sometimes used to write really tight code (really discouraged, code not maintainable)

2. Microprocessor Architecture
ADD R0,NUM
Microprocessor Architecture
Instruction Fetch Execute
1. MAR<-PC, read 7. alu<-R0, add
2. wait, PC<-PC+1 Write Back
3. IR <- MDR 8. MDR<-buffer
Instruction Decode 9. MAR<- ‘NUM’,
4. MAR <- ’NUM’, read write
5. Wait wait
6. Acc <- MDR
Single bus architecture, instruction execution (simplified!) M D R
Accumulator IR SP
MAR PC Rn … R0
ALU
CCR
buffer

3. 1006 223A FFFA move.l start+2(pc),d1
code/data reuse
Objective: obscure value in D1
ORG $1000
start:
C move.l #$ ,d0
A FFFA move.l start+2(pc),d1
100A 4EB xxx jsr dobasedond1
...
1xxx 4E stop #$2700
1xxx dobasedond1:
1xxx 4E rts
END start

4. 1000 4EBA 000E entry: jsr label2(pc) 1004 4EBA 0006 jsr label(pc)
self modifying code
ORG $1000
EBA 000E entry: jsr label2(pc)
EBA jsr label(pc)
E stop #$2700
100C label:
100C 4E rts
100E 4E nop ← code of interest here
FA FFFA label2: lea label(pc),a0
BC 4E move.w #$4e71,(a0)
E rts
101A END entry

5. data dc.l $13fc01ff, $00e0110,$533901e0,$001067f8,$4e722600
self modifying code
START ORG $1000
F lea data,a0
E3C 00000FFF move.l #4095,d7
100C 203C move.l #$100,d0
1012 B loop: eor.l d0,(a0)+
CF FFFC dbra d7,loop
bra data
101C 4E stop #$2700
1020= 13FC01FF 000E
data dc.l $13fc01ff, $00e0110,$533901e0,$001067f8,$4e722600
END START

6. 1008 41FA 000A time1 lea label+2(pc),a0 100C 3210 move.w (a0),d1
self modifying code
START ORG $1000
C move.w #$0004,d2
C move.w #5,d0
FA 000A time1 lea label+2(pc),a0
100C move.w (a0),d1
100E B eor.w d2,d1
C move.w d1,(a0)+
C8 FFF4 label dbra d0,time1
1016 4E stop #$2700
END START