B BRANCH ON (CONDTION)
|
Purpose |
test condition code |
|
Format |
opcode displ.bb |
|
Operation |
if condition then PC ß PC + SEXT (displ); |
|
Condition codes |
N ß N; Z ß Z; V ß V; C ß C; |
|
Exceptions |
none |
|
Opcodes |
CONDITION 12 Z EQL 0 BNEQ, Branch on Not Equal (signed) BNEQU, Branch on Not Equal Unsigned 13 Z EQL 1 BEQL, Branch on Equal (signed) BEOLU, Branch on Equal Unsigned 14 {N OR Z}EQL 0 BGTR, Branch on Greater Than (signed) 15 {N OR Z}EQL 1 BLEQ, Branch on Less Than or Equal (signed) 18 N EQL 0 BGEQ, Branch on Greater Than or Equal (signed) 19 N EQL 1 BLSS, Branch on Less Than (signed) 1A {C OR Z}EQL 0 BGTRU, Branch on Greater Than Unsigned 1B {C OR Z}EQL 1 BLEQU, Branch Less Than or Equal Unsigned 1C V EQL 0 BVC, Branch on Overflow Clear 1D V EQL 1 BVS, Branch on Overflow Set 1E C EQL 0 BGEQU, Branch on Greater Than or Equal unsigned BCC Branch on Carry Clear 1F C EQL 1 BLSSU, Branch on Less Than Unsigned BSC Branch on Carry Set |
|
Description |
The condition codes are tested and if the condition indicated by the instruction is met, the sign-extended branch displacement is added to the PC and PC is replaced by the result. |
|
Notes |
The VAX- conditional branch instructions permit considerable flexibility in branching but require care in choosing the correct branch instruction. The conditional branch instructions are divided into 3 overlapping groups: 1. Overflow and Carry Group BVS V EQL 1 BVC V EQL 0 BCS C EQL 1 BCC C EQL 0 These instructions are typically used to check for overflow (when overflow traps are not enabled), for multiprecision arithmetic, and for other special purposes. 2. Unsigned Group BLSSU C EQL 1 BLEQU {C or Z} EQL 1 BEQLU Z EQL 1 BNEQU Z EQL 0 BGEQU C EOL 0 BGTRU {C OR Z} EQL 0 These instructions typically follow integer and field in structions where the operands are treated as unsigned integers, addressed instructions, and character string in structions. 3. Signed Group BLSS N EQL 1 BLEQ {N OR Z} EQL 1 BEQL Z EQL 1 BNEQ Z EQL 0 BGEQ N EQL 0 BGTR {N OR Z} EQL 0 These instructions typically follow integer and field instructions where the operands are being treated as signed integers, floating point instructions, and decimal string instructions. |
Example 1 - BEQL
The following program shows the usage of BEQL opcode.
.text
main: .word 0
# first case - Z should be 1 (Equal)
movb $0, r1
beql eq1
calls $0, prn_not_eq
jmp next_stage
eq1: calls $0, prn_eq
# second case - Z should be 0 (Not Equal)
next_stage:
movb $1, r1
beql eq2
calls $0, prn_not_eq
jmp end_prog
eq2: calls $0, prn_eq
end_prog:
pushl $0
calls $1, .exit
prn_not_eq: .word 0
pushal not_eq
pushal format
calls $2, .printf
ret
prn_eq: .word 0
pushal eq
pushal format
calls $2, .printf
ret
.data
eq: .asciz "Equal"
not_eq: .asciz "Not Equal"
format: .asciz "%s\n"
Example 2 – BNEQ, BNEQU
Almost the same as the previous example, this program shows the usage of BNEQ, BNEQU opcodes.
.text
main: .word 0
# first case - Z should be 0 (Not Equal)
movb $0, r1
bneq eq1
calls $0, prn_not_eq
jmp next_stage
eq1: calls $0, prn_eq
# second case - Z should be 1 (Equal)
next_stage:
movb $1, r1
bnequ eq2
calls $0, prn_not_eq
jmp end_prog
eq2: calls $0, prn_eq
end_prog:
pushl $0
calls $1, .exit
prn_not_eq: .word 0
pushal not_eq
pushal format
calls $2, .printf
ret
prn_eq: .word 0
pushal eq
pushal format
calls $2, .printf
ret
.data
eq: .asciz "Equal"
not_eq: .asciz "Not Equal"
format: .asciz "%s\n"
Example 3 – BGTR
.text
main: .word 0
movb $10, r1
movb $5, r2
# first case - should be True
cmpb r1, r2
bgtr eq1
calls $0, prn_false
jmp next_stage
eq1: calls $0, prn_true
# second case - should be False
next_stage:
cmpb r2, r1
bgtr eq2
calls $0, prn_false
jmp end_prog
eq2: calls $0, prn_true
end_prog:
pushl $0
calls $1, .exit
prn_false: .word 0
pushal lbl_false
pushal format
calls $2, .printf
ret
prn_true: .word 0
pushal lbl_true
pushal format
calls $2, .printf
ret
.data
lbl_true: .asciz "True"
lbl_false: .asciz "False"
format: .asciz "%s\n"
Example 4 – BGEQ
.text
main: .word 0
movb $10, r1
movb $5, r2
# first case - should be True
cmpb r1, r2
bgeq eq1
calls $0, prn_false
jmp next_stage
eq1: calls $0, prn_true
# second case - should be False
next_stage:
cmpb r2, r1
bgeq eq2
calls $0, prn_false
jmp end_prog
eq2: calls $0, prn_true
end_prog:
pushl $0
calls $1, .exit
prn_false: .word 0
pushal lbl_false
pushal format
calls $2, .printf
ret
prn_true: .word 0
pushal lbl_true
pushal format
calls $2, .printf
ret
.data
lbl_true: .asciz "True"
lbl_false: .asciz "False"
format: .asciz "%s\n"