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This is our program, as it was, I've left the NOP in it as to not change our initial results from chapter 13, also we will have to replace RTS with BRK to accurately count cycles on virtual 6502.
; C64 Hex to Binary display converter
; 64TASS assembler style code for 6502
; Jordan Rubin 2014 http://technocoma.blogspot.com
;
; Takes the HEX value in OURHEXVAUE, converts it to Binary for display
; on the screen as a binary number.
*=$C000 ; SYS 49152 to begin
OURHEXVALUE = #$55 ; Enter the Hex value to be converted here
OURHEXNUM = $033C ; This is where the constant OURHEXVALUE will be stored
TESTBYTE = $0345 ; This is where our test byte will be stored for lsr
BIT7 = $0708 ; This is the location of the 7th bit, required room for
; 8 contiguous bytes after the starting address
; using 0708 dumps it right to screen ram, bottom center
nop
INIT:
lda OURHEXVALUE ; this will be out test number
sta OURHEXNUM ; we will store the test number here perminantly
ldy #$80 ; Out first bit test for bit 7 must be 10000000 $80
sty TESTBYTE ; store our initial test byte here
ldx #$00 ; Initialize X for our loop
CONVERTION:
lda OURHEXNUM ; load our test hex number, this is a constant
and TESTBYTE ; mask it with our test byte
cmp #$00 ; is the result 00?
bne STORE1 ; No, jsr to STORE1
beq STORE0 ; Yes, jsr to STORE0
CONTINUE:
inx ; Increment X for our loop
lda TESTBYTE ; load testbyte into A
lsr ; divide it by 2
sta TESTBYTE ; store new testbyte back to its memory area
cpx #$08 ; is X=8?
bne CONVERTION ; No, LOOP back to CONVERSION
brk
STORE0:
lda #$30 ; Load the display value of 0 into A
sta BIT7,x ; store A to the current storage memory location
jmp CONTINUE ; jump to CONTINUE
STORE1:
lda #$31 ; Load the display value of 0 into A
sta BIT7,x ; store A to the current storage memory location
jmp CONTINUE ; jump to CONTINUE
Writing optimized code could be daunting, maybe its better to write working code. Such as that above and then optimize it.
OUR current code, as shown occupies memory area C000 to C035. [54 bytes]
Executing it in virtual 6502 shows it required 349 Cycles to complete
ea a9 55 8d 3c 03 a0 80 8c 45 03 a2 00 ad 3c 03 2d 45 03 c9 00 d0 17 f0 0d e8 ad 45 03 4a 8d 45 03 e0 08 d0 e8 60 a9 30 9d 08 07 4c 19 c0 a9 31 9d 08 07 4c 19 c0
Lets see if we can improve upon this, its not a big program, and there isn't much to do, but there is enough to do..... remember we can save a byte and a cycle getting rid of NOP, but well keep it so we can view the code in the monitor, lets move to the real stuff
INIT:
This is rather straight forward, and no waste,we'll leave it alone
STORE0: and STORE1:
Both seem to have the instruction stay BIT7,x just before jumping to CONTINUE. why not move that instruction into CONTINUE to the top line, both STORE0 and STORE1 will execute it anyway. This won't save us any cycles, but it will save us some space
[OLD]
CONVERTION:
lda OURHEXNUM ; load our test hex number, this is a constant
and TESTBYTE ; mask it with our test byte
cmp #$00 ; is the result 00?
bne STORE1 ; No, jsr to STORE1
beq STORE0 ; Yes, jsr to STORE0
CONTINUE:
inx ; Increment X for our loop
lda TESTBYTE ; load testbyte into A
lsr ; divide it by 2
sta TESTBYTE ; store new testbyte back to its memory area
cpx #$08 ; is X=8?
bne CONVERTION ; No, LOOP back to CONVERSION
brk
STORE0:
lda #$30 ; Load the display value of 0 into A
sta BIT7,x ; store A to the current storage memory location
jmp CONTINUE ; jump to CONTINUE
STORE1:
lda #$31 ; Load the display value of 0 into A
sta BIT7,x ; store A to the current storage memory location
jmp CONTINUE ; jump to CONTINUE
[NEW]
CONVERTION:
lda OURHEXNUM ; load our test hex number, this is a constant
and TESTBYTE ; mask it with our test byte
cmp #$00 ; is the result 00?
bne STORE1 ; No, jsr to STORE1
beq STORE0 ; Yes, jsr to STORE0
CONTINUE:
sta BIT7,x ; store A to the current storage memory location
inx ; Increment X for our loop
lda TESTBYTE ; load testbyte into A
lsr ; divide it by 2
sta TESTBYTE ; store new testbyte back to its memory area
cpx #$08 ; is X=8?
bne CONVERTION ; No, LOOP back to CONVERSION
brk
STORE0:
lda #$30 ; Load the display value of 0 into A
jmp CONTINUE ; jump to CONTINUE
STORE1:
lda #$31 ; Load the display value of 0 into A
jmp CONTINUE ; jump to CONTINUE
Now that this was done, lets look further at our code. We can see in conversion that two possible branches exist STORE0 or STORE1
CONVERTION:
lda OURHEXNUM ; load our test hex number, this is a constant
and TESTBYTE ; mask it with our test byte
cmp #$00 ; is the result 00?
bne STORE1 ; No, jsr to STORE1
beq STORE0 ; Yes, jsr to STORE0
If there are only 2 possibilities it seems like a waste to jsr to both based on our tests when we can have a normal program flow and throw 1 exception
Why not keep our exception bne STORE1 and put our STORE0 code right below to just continue on.
Well have to move the code so that STORE0 is directly under CONVERSION
CONVERTION:
lda OURHEXNUM ; load our test hex number, this is a constant
and TESTBYTE ; mask it with our test byte
cmp #$00 ; is the result 00?
bne STORE1 ; No, jsr to STORE1
lda #$30 ; Load the display value of 0 into A
jmp CONTINUE ; jump to CONTINUE
STORE1:
lda #$31 ; Load the display value of 1 into A
CONTINUE:
sta BIT7,x ; Load the display value into A
inx ; Increment X for our loop
lda TESTBYTE ; load testbyte into A
lsr ; divide it by 2
sta TESTBYTE ; store new testbyte back to its memory area
cpx #$08 ; is X=8?
bne CONVERTION ; No, LOOP back to CONVERSION
brk
Essentially there is no more STORE0 function. In our new program flow A will be $30 unless it branched to STORE1, which makes A $31. Both CONVERSION and STORE1 ultimately lead to CONTINUE.
Lets look at the final optimized program, we kept the no in for a fair comparison between the old and new code
; C64 Hex to Binary display converter optimized
; 64TASS assembler style code for 6502
; Jordan Rubin 2014 http://technocoma.blogspot.com
;
; Takes the HEX value in OURHEXVAUE, converts it to Binary for display
; on the screen as a binary number.
*=$C000 ; SYS 49152 to begin
OURHEXVALUE = #$55 ; Enter the Hex value to be converted here
OURHEXNUM = $033C ; This is where the constant OURHEXVALUE will be stored
TESTBYTE = $0345 ; This is where our test byte will be stored for lsr
BIT7 = $0708 ; This is the location of the 7th bit, required room for
; 8 contiguous bytes after the starting address
; using 0708 dumps it right to screen ram, bottom center
nop
INIT:
lda OURHEXVALUE ; this will be out test number
sta OURHEXNUM ; we will store the test number here permanently
ldy #$80 ; Out first bit test for bit 7 must be 10000000 $80
sty TESTBYTE ; Store our initial test byte here
ldx #$00 ; Initialize X for our loop
CONVERTION:
lda OURHEXNUM ; load our test hex number, this is a constant
and TESTBYTE ; mask it with our test byte
cmp #$00 ; is the result 00?
bne STORE1 ; No, jsr to STORE1
lda #$30 ; Load the display value of 0 into A
jmp CONTINUE ; jump to CONTINUE
STORE1:
lda #$31 ; Load the display value of 1 into A
CONTINUE:
sta BIT7,x ; Load the display value into A
inx ; Increment X for our loop
lda TESTBYTE ; load testbyte into A
lsr ; divide it by 2
sta TESTBYTE ; store new testbyte back to its memory area
cpx #$08 ; is X=8?
bne CONVERTION ; No, LOOP back to CONVERSION
brk
OUR current code, as shown occupies memory area C000 to C02D. [46 bytes]
Executing it in virtual 6502 shows it required 319 Cycles to complete
ea a9 55 8d 3c 03 a0 80 8c 45 03 a2 00 ad 3c 03 2d 45 03 c9 00 d0 05 a9 30 4c 1e c0 a9 31 9d 08 07 e8 ad 45 03 4a 8d 45 03 e0 08 d0 e0 00
We put our code in and change the start address. Than click load memory.
Then we click show memory and click the green PC and change it to C000
Clicking continuous run we see how many cycles are required before it breaks
So completing the same function we went from
[54 bytes] 349 Cycles to complete
to
[46 bytes] 319 Cycles to complete
Reducing the code size by 8 bytes and reducing the processor cycles by 30. (Almost 15%)
NEXT----->
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