7.4. Calling Member FunctionsΒΆ
Calling member functions is no different than calling any other function in
assembly. The only special setup that needs to be done is to load into r0
the address of the object that will be this while executing the code.
The sample below sets up a minimal stack frame that has just space for two
Time objects (it does not store lr or fp). time1 will occupy
the 12 bytes that fp points at and time2 will occupy the 12 bytes
starting at fp - 12. Any time a constructor or member function needs
to be called, one of those two addresses will be loaded into r0:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 | /*
Time class structure:
12 bytes total space
address - 8 : hour
address - 4 : minute
address - 0 : second
Stack in main:
-------------------------------
FramePointer
Relative
Address Contents
-------------------------------
-20 time2.hour
-16 time2.minute
-12 time2.second
-8 time1.hour
-4 time1.minute
0 time1.second
*/
.text
_start:
@allocate space on the stack for two Time objects
SUB fp, sp, #4 @fp will point to block of memory where two Times are
SUB sp, sp, #24 @allocate stack space for two Times
@time1 will be at fp, time2 will be at fp+12
@construct Time time1
MOV r0, fp @load time1's address into r0
BL Time_Time
@time1.setMinute(5)
MOV r0, fp @load time1's address into r0
MOV r1, #5 @load 1st parameter into r1
BL Time_setMinute
@r4 = time1.getMinute()
MOV r0, fp @load time1's address into r0
BL Time_getMinute
MOV r4, r0
@construct Time time2(22, 45, 10)
SUB r0, fp, #12 @load time2's address into r0
MOV r1, #22 @load 1st parameter into r1
MOV r2, #45 @load 2nd parameter into r2
MOV r3, #10 @load 3rd parameter into r3
BL Time_Time_int_int_int
@time2.incrHour() x3
SUB r0, fp, #12 @load time2's address into r0
BL Time_incrHour @22->23
BL Time_incrHour @23->00
BL Time_incrHour @00->1
@r5 = time2.getHour()
SUB r0, fp, #12 @load time2's address into r0
BL Time_getHour
MOV r5, r0
@deallocate stack space
ADD sp, sp, #24
end:
B end
/*
Time()
Condensed version... does not worry about building stack frame
Params:
r0 = address of this
*/
Time_Time:
@store any appropriate registers
@r0 has address of this object
MOV r1, #0
STR r1, [r0, #0] @this->second = 0
STR r1, [r0, #-4] @this->minute = 0
STR r1, [r0, #-8] @this->hour = 0
@restore stack/registers and return
BX lr
/*
Condensed version... does not worry about building stack frame
Time::Time(int h, int m, int s)
Params:
r0 = address of this
r1 = h
r2 = m
r3 = s
*/
Time_Time_int_int_int:
@store any appropriate registers
@r0 has address of this object
STR r3, [r0, #0] @this->second = s
STR r2, [r0, #-4] @this->minute = m
STR r1, [r0, #-8] @this->hour = h
@restore stack/registers and return
BX lr
/*
Condensed version... does not worry about building stack frame
int Time::getMinute()
Params:
r0 = address of this
*/
Time_getMinute:
@store any appropriate registers
@r0 has address of this object
LDR r1, [r0, #-4] @r1 = this->minute
@place return value in r0
MOV r0, r1
@restore stack/registers and return
BX lr
/*
Condensed version... does not worry about building stack frame
void Time::setMinute(int m)
Params:
r0 = address of this
r1 = m
*/
Time_setMinute:
@store any appropriate registers
@r0 has address of this object, minute is 4 bytes into this
STR r1, [r0, #-4] @this->minute = r1
@restore stack/registers and return
BX lr
/*
Condensed version... does not worry about building stack frame
void Time::incrHour()
Params:
r0 = address of this
*/
Time_incrHour:
@store any appropriate registers
@r0 has address of this object
LDR r1, [r0, #-8] @r1 = this->hour
@update hour
ADD r1, r1, #1
CMP r1, #24 @check if == 24
MOVEQ r1, #0 @if so, roll over to 0
@store back to memory
STR r1, [r0, #-8] @this->hour = r1
@restore stack/registers and return
BX lr
/*
Condensed version... does not worry about building stack frame
int Time::getHour()
Params:
r0 = address of this
*/
Time_getHour:
@store any appropriate registers
@r0 has address of this object
LDR r1, [r0, #-8] @r1 = this->hour
@place return value in r0
MOV r0, r1
@restore stack/registers and return
BX lr
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