1. CS-401 Computer Architecture & Assembly Language Programming
Lecture-8
Addressing Modes
Branching

2. In the Last Lecture
We discussed memory addresses

3. Addressing Modes Offset Addressing ; Default segment = ds
mov ax, [0x1234] ; word move
mov ah, [0x1234] ; byte move
mov byte[0x1234], 10
mov word[0x1234], 10

4. Addressing Modes Base Addressing ; Default segment = ds mov ax, [bx]
mov byte [bx], 10
mov word [bx], 10
; Default segment = ss
mov ax, [bp]
mov byte [bp], 10
mov word [bp], 10

5. Addressing Modes Index Addressing ; Default segment = ds mov ax, [si]
mov byte [di], 10
mov word [si], 10

6. Base + Offset Addressing
Addressing Modes
Base + Offset Addressing
; Default segment = ds
mov ax, [bx+0x0100]
mov byte [bx+0x100], 10
mov word [bx+0x100], 10
; Default segment = ss
mov byte [bp+0x10], 10
mov word [bp+0x100], 10

7. Index + Offset Addressing
Addressing Modes
Index + Offset Addressing
; Default segment = ds
mov ax, [si+0x0100]
mov byte [di+0x100], 10
mov word [di+0x100], 10
mov byte [si+0x10], 10
mov word [si+0x0100], 10

8. Base + Index Addressing
Addressing Modes
Base + Index Addressing
; Default segment = ds
mov ax, [bx+si]
mov byte [bx+si], 10
mov word [bx+di], 10
; Default segment = ss
mov byte [bp+si], 10
mov word [bp+di], 10

9. Base + Index + Offset Addressing
Addressing Modes
Base + Index + Offset Addressing
; Default segment = ds
mov ax, [bx+si+0x0100]
mov byte [bx+si+0x0100], 10
mov word [bx+di+0x0100], 10
; Default segment = ss
mov byte [bp+si+0x0100], 10
mov word [bp+di+0x0100], 10

10. Addressing Modes
General Form
[base + index + offset]

11. Illegal Addressing Example mov al, [bl] ; Address cannot be 8-bit
; has to be 16-bit

12. Illegal addressing

13. Illegal Addressing Example
mov ax, [bx-si] ; Registers cannot be ; subtracted

14. Illegal Addressing Example
mov ax, [bx+bp] ; Two bases cannot be ; used in one addressing

15. Illegal Addressing Example
mov ax, [si+di] ; Two indices cannot be ; used in one addressing

16. Physical Address Calculation
[cs:bx+si+0x0700] BX = 0x0100
SI = 0x0200
CS = 0x1000
Effective Address =
EA = Base + Index + Offset
EA = 0x x x0700
= 0x0A00
Physical Address = Segment*0x10+EA
= 0x1000*0x10+0xA00
= 0x10A00

17. Physical Address Calculation
[bx+0x7000] BX = 0x9100
DS = 0x1500
Effective Address =
EA = Base + Index + Offset
= 0x x x07000
= 0x10100 ; 17-bits !
= 0x0100 ; Segment wrap ; around

18. Physical Address Calculation
[bx+0x7000] BX = 0x9100
DS = 0x1500
Effective Address =
EA = 0x0100 ; Segment wrap ; around
Physical Address = Segment*0x10+EA
= 0x1500*0x10+0x100
= 0x15100

19. Physical Address Calculation
[bx+0x0100] BX = 0x0100
DS = 0xFFF0
Effective Address =
EA = Base + Index + Offset
= 0x x x0100
= 0x0200
Physical Address = Segment*0x10+EA
= 0xFFF0*0x10+0x0200
= 0x ; 21-bits !
= 0x ; memory wrap ; around

20. CMP Subtracts the source (src) from destination (dest)
Does not change contents of src or dest.
Affects AF,CF,OF,PF,SF and ZF.
The relation is of destination to source.

21. Conditional Jumps jz ;jump if zero [ZF=1] Jnz ;jump if not zero [ZF=0]
Example
cmp ax, bx
jz label1

22. Conditional Jumps je ;jump if equal [same as jz]
Jne ;jump if not equal [same as jnz]

23. Conditional Jumps jc ;jump if carry [CF=1]
Jnc ;jump if not carry [CF=0]
Example
add ax, bx
jc label1
sub ax, bx
jnc label1

24. Conditional Jumps ja ;jump if above [ZF = 0 and CF=0]
jb ;jump if below [CF=1]
unsigned integers
jl ;jump if less [SF <> OF=0]
jg ;jump if greater [ZF = 0 and SF=OF]
signed integers

25. Conditional Jumps jae ;jump if above or equal
jbe ;jump if below or equal
jge ;jump if greater or equal
jle ;jump if less or equal
jno ;Jump if not overflow
jns ; Jump if not sign

26. Renamed Conditional Jumps
JNBE  JA
JNB  JAE
JNAE  JB
JNA  JBE
JZ  JE
JNLE  JG
JNL  JGE
JNGE  JL
JNG  JLE
JNZ  JNE
JPO  JNP
JPE  JP

27. Conditional Jumps
jcxz ;jump if the cx register is zero [cx=0]