Understanding 6502 assembly on the Commodore 64 - (13) Watching the 6502 Think

I wrote a small program today, It takes a HEX value and returns the binary value onto the screen, or any other contiguous memory location.  I kept it simple, and while simple, still highly effective and adaptable.  What is more interesting though, is stepping through each process as the processor works towards the end of the program.  Thats what were going to do.  This is very hardware neutral code, Changing OURHEXNUM,TESTBYTE, and BIT7 to your preferred memory area will allow this to run on any 6502 based computer.









Heres the program..............

; C64 Hex to Binary display converter
; 64TASS assembler style code for 6502
; Jordan Rubin 2014 http://technocoma.blogspot.com
;
; Takes the HEX value in OURHEXVALUE, 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                ; added only to pad the monitor in vice for this demo

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
rts

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


I could optimize it further, but for now it looks rather elegant.  I wrote this for an easy display of some outputs I need off of a serial interface control.  We'll work with this more in the future, for now, lets run it!!!!

You'll see that at the top of the program, OURHEXVALUE was $55.  You can see upon execution the binary value of $55 is displayed on the screen.  Changing OURHEXVALUE will yield a different result when the program is executed.  I know, it seems pretty useless in this form.  In its actual form,

Loading A with the HEX value
Loading X with the MSB of the output memory location bit 7, and
Loading Y with the LSB of the output memory location bit 7
JSR to beginning of where you store this program

We just created a kernel level function for another program to use within the computer!!!!!!

OK so, I want to show you a nifty trick, this is the basis for this whole chapter

With the screen showing our result, go up to your menu, to machine, and select monitor, bringing up the monitor is really unimpressive, I know.....  lets see what amazing things we can to here.  Our program has executed and ended lets re-run the program in the monitor and see what happens with our value $55

first we need to move the program counter to the memory area of our program 

r pc=c000

then we type step

I added a function to the beginning NOP,   it does nothing.... literally nothing, but its existence allowed me to pad the monitor to step nicely from C000.  With our monitor lets watch the program run from beginning to end,  Ill highlight the interesting parts, pressing enter will execute 1 instruction at at time

Remember, we will not see our LABELS here, the processor uses the actual memory addresses from the code we wrote.

Remember:

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


I didn't remark everything, the first two iterations are remarked. there after I pointed to certain areas to make the code easier to follow.  Note the BIT 1 or our flags, the Z bit Z and the .  when values are tested.  This controls the flow of the whole program!

A move of the process is included below

(C:$e5d4) r pc=c000

(C:$e5d4) step

.C:c001  A9 55       LDA #$55       - A:00 X:00 Y:0A SP:f2 ..-...Z.   10476655

(C:$c001) 

.C:c003  8D 3C 03    STA $033C      - A:55 X:00 Y:0A SP:f2 ..-.....   10476657

(C:$c003) 

.C:c006  A0 80       LDY #$80       - A:55 X:00 Y:0A SP:f2 ..-.....   10476661

(C:$c006) 

.C:c008  8C 45 03    STY $0345      - A:55 X:00 Y:80 SP:f2 N.-.....   10476663

(C:$c008) 

.C:c00b  A2 00       LDX #$00       - A:55 X:00 Y:80 SP:f2 N.-.....   10476667
(C:$c00b) X counter is 0, A is our convert number  TESTYBYE is stored as 10000000
.C:c00d  AD 3C 03    LDA $033C      - A:55 X:00 Y:80 SP:f2 ..-...Z.   10476669

(C:$c00d) 

.C:c010  2D 45 03    AND $0345      - A:55 X:00 Y:80 SP:f2 ..-.....   10476673

(C:$c010) 

.C:c013  C9 00       CMP #$00       - A:00 X:00 Y:80 SP:f2 ..-...Z.   10476677  

(C:$c013)The AND turns A to 00 if 0 or anything else if 1 

.C:c015  D0 17       BNE $C02E      - A:00 X:00 Y:80 SP:f2 ..-...ZC   10476679  
(C:$c015) If not 00 jump to C02E, but it is, ignore
.C:c017  F0 0D       BEQ $C026      - A:00 X:00 Y:80 SP:f2 ..-...ZC   10476681

(C:$c017) If is 00 jump to C026 

.C:c026  A9 30       LDA #$30       - A:00 X:00 Y:80 SP:f2 ..-...ZC   10476684

(C:$c026) Here we are at C026, load A the screen value of 0, $#30 

.C:c028  9D 08 07    STA $0708,X    - A:30 X:00 Y:80 SP:f2 ..-....C   10476686

(C:$c028) Store it to location BIT7+0 

.C:c02b  4C 19 C0    JMP $C019      - A:30 X:00 Y:80 SP:f2 ..-....C   10476691
(C:$c02b) Jump to CONTINUE label
.C:c019  E8          INX            - A:30 X:00 Y:80 SP:f2 ..-....C   10476694
(C:$c019) Make X, or cycle counter 1, X=X+1
.C:c01a  AD 45 03    LDA $0345      - A:30 X:01 Y:80 SP:f2 ..-....C   10476696
(C:$c01a) Load TESTBYE into A
.C:c01d  4A          LSR A          - A:80 X:01 Y:80 SP:f2 N.-....C   10476700
(C:$c01d) Divide A by 2
.C:c01e  8D 45 03    STA $0345      - A:40 X:01 Y:80 SP:f2 ..-.....   10476702
(C:$c01e) Store our new value in TESTBYTE
.C:c021  E0 08       CPX #$08       - A:40 X:01 Y:80 SP:f2 ..-.....   10476706
(C:$c021) Does X = 8?
.C:c023  D0 E8       BNE $C00D      - A:40 X:01 Y:80 SP:f2 N.-.....   10476708
(C:$c023) NO? ok jump to beginning of our loop 
.C:c00d  AD 3C 03    LDA $033C      - A:40 X:01 Y:80 SP:f2 N.-.....   10476711

(C:$c00d) 

.C:c010  2D 45 03    AND $0345      - A:55 X:01 Y:80 SP:f2 ..-.....   10476715

(C:$c010) X counter is 1, A is our convert value, TESTYBYE is stored as 01000000

.C:c013  C9 00       CMP #$00       - A:40 X:01 Y:80 SP:f2 ..-.....   10476719

(C:$c013) The AND turns A to 00 if 0 or anything else if 1

.C:c015  D0 17       BNE $C02E      - A:40 X:01 Y:80 SP:f2 ..-....C   10476721

(C:$c015) If not 00 jump to C02E, so we jump

.C:c02e  A9 31       LDA #$31       - A:40 X:01 Y:80 SP:f2 ..-....C   10476724

(C:$c02e) Here we are at C026, load A the screen value of 1, $#31

.C:c030  9D 08 07    STA $0708,X    - A:31 X:01 Y:80 SP:f2 ..-....C   10476726

(C:$c030) Store it to location BIT7+1, because x is now 1

.C:c033  4C 19 C0    JMP $C019      - A:31 X:01 Y:80 SP:f2 ..-....C   10476731

(C:$c033) 

.C:c019  E8          INX            - A:31 X:01 Y:80 SP:f2 ..-....C   10476734
(C:$c019) Make X, or cycle counter 2, X=X+1
.C:c01a  AD 45 03    LDA $0345      - A:31 X:02 Y:80 SP:f2 ..-....C   10476736

(C:$c01a) Load TESTBYE into A

.C:c01d  4A          LSR A          - A:40 X:02 Y:80 SP:f2 ..-....C   10476740

(C:$c01d) Divide A by 2

.C:c01e  8D 45 03    STA $0345      - A:20 X:02 Y:80 SP:f2 ..-.....   10476742

(C:$c01e) Store our new value in TESTBYTE

.C:c021  E0 08       CPX #$08       - A:20 X:02 Y:80 SP:f2 ..-.....   10476746

(C:$c021) Does X = 8?

.C:c023  D0 E8       BNE $C00D      - A:20 X:02 Y:80 SP:f2 N.-.....   10476748

(C:$c023) NO? ok jump to beginning of our loop

.C:c00d  AD 3C 03    LDA $033C      - A:20 X:02 Y:80 SP:f2 N.-.....   10476751

(C:$c00d) 

.C:c010  2D 45 03    AND $0345      - A:55 X:02 Y:80 SP:f2 ..-.....   10476755

(C:$c010) X counter is 2, A is our convert value, TESTYBYE is stored as 00100000

.C:c013  C9 00       CMP #$00       - A:00 X:02 Y:80 SP:f2 ..-...Z.   10476759

(C:$c013) 

.C:c015  D0 17       BNE $C02E      - A:00 X:02 Y:80 SP:f2 ..-...ZC   10476761

(C:$c015) 

.C:c017  F0 0D       BEQ $C026      - A:00 X:02 Y:80 SP:f2 ..-...ZC   10476763

(C:$c017) 

.C:c026  A9 30       LDA #$30       - A:00 X:02 Y:80 SP:f2 ..-...ZC   10476766

(C:$c026) 

.C:c028  9D 08 07    STA $0708,X    - A:30 X:02 Y:80 SP:f2 ..-....C   10476768

(C:$c028) 

.C:c02b  4C 19 C0    JMP $C019      - A:30 X:02 Y:80 SP:f2 ..-....C   10476773

(C:$c02b) 

.C:c019  E8          INX            - A:30 X:02 Y:80 SP:f2 ..-....C   10476776
(C:$c019) Make X, or cycle counter 3, X=X+1
.C:c01a  AD 45 03    LDA $0345      - A:30 X:03 Y:80 SP:f2 ..-....C   10476778

(C:$c01a) 

.C:c01d  4A          LSR A          - A:20 X:03 Y:80 SP:f2 ..-....C   10476782

(C:$c01d) 

.C:c01e  8D 45 03    STA $0345      - A:10 X:03 Y:80 SP:f2 ..-.....   10476784

(C:$c01e) 

.C:c021  E0 08       CPX #$08       - A:10 X:03 Y:80 SP:f2 ..-.....   10476788

(C:$c021) 

.C:c023  D0 E8       BNE $C00D      - A:10 X:03 Y:80 SP:f2 N.-.....   10476790

(C:$c023) 

.C:c00d  AD 3C 03    LDA $033C      - A:10 X:03 Y:80 SP:f2 N.-.....   10476793

(C:$c00d) 

.C:c010  2D 45 03    AND $0345      - A:55 X:03 Y:80 SP:f2 ..-.....   10476797

(C:$c010) X counter is 3, A is our convert value, TESTYBYE is stored as 00010000

.C:c013  C9 00       CMP #$00       - A:10 X:03 Y:80 SP:f2 ..-.....   10476801

(C:$c013) 

.C:c015  D0 17       BNE $C02E      - A:10 X:03 Y:80 SP:f2 ..-....C   10476803

(C:$c015) 

.C:c02e  A9 31       LDA #$31       - A:10 X:03 Y:80 SP:f2 ..-....C   10476806

(C:$c02e) 

.C:c030  9D 08 07    STA $0708,X    - A:31 X:03 Y:80 SP:f2 ..-....C   10476808

(C:$c030) 

.C:c033  4C 19 C0    JMP $C019      - A:31 X:03 Y:80 SP:f2 ..-....C   10476813

(C:$c033) 

.C:c019  E8          INX            - A:31 X:03 Y:80 SP:f2 ..-....C   10476816
(C:$c019) Make X, or cycle counter 4, X=X+1
.C:c01a  AD 45 03    LDA $0345      - A:31 X:04 Y:80 SP:f2 ..-....C   10476818

(C:$c01a) 

.C:c01d  4A          LSR A          - A:10 X:04 Y:80 SP:f2 ..-....C   10476822

(C:$c01d) 

.C:c01e  8D 45 03    STA $0345      - A:08 X:04 Y:80 SP:f2 ..-.....   10476824

(C:$c01e) 

.C:c021  E0 08       CPX #$08       - A:08 X:04 Y:80 SP:f2 ..-.....   10476828

(C:$c021) 

.C:c023  D0 E8       BNE $C00D      - A:08 X:04 Y:80 SP:f2 N.-.....   10476830

(C:$c023) 

.C:c00d  AD 3C 03    LDA $033C      - A:08 X:04 Y:80 SP:f2 N.-.....   10476833

(C:$c00d) 

.C:c010  2D 45 03    AND $0345      - A:55 X:04 Y:80 SP:f2 ..-.....   10476837

(C:$c010) X counter is 4, A is our convert value, TESTYBYE is stored as 00001000

.C:c013  C9 00       CMP #$00       - A:00 X:04 Y:80 SP:f2 ..-...Z.   10476841

(C:$c013) 

.C:c015  D0 17       BNE $C02E      - A:00 X:04 Y:80 SP:f2 ..-...ZC   10476843

(C:$c015) 

.C:c017  F0 0D       BEQ $C026      - A:00 X:04 Y:80 SP:f2 ..-...ZC   10476845

(C:$c017) 

.C:c026  A9 30       LDA #$30       - A:00 X:04 Y:80 SP:f2 ..-...ZC   10476848

(C:$c026) 

.C:c028  9D 08 07    STA $0708,X    - A:30 X:04 Y:80 SP:f2 ..-....C   10476850

(C:$c028) 

.C:c02b  4C 19 C0    JMP $C019      - A:30 X:04 Y:80 SP:f2 ..-....C   10476855

(C:$c02b) 

.C:c019  E8          INX            - A:30 X:04 Y:80 SP:f2 ..-....C   10476858
(C:$c019) Make X, or cycle counter 5, X=X+1
.C:c01a  AD 45 03    LDA $0345      - A:30 X:05 Y:80 SP:f2 ..-....C   10476860

(C:$c01a) 

.C:c01d  4A          LSR A          - A:08 X:05 Y:80 SP:f2 ..-....C   10476864

(C:$c01d) 

.C:c01e  8D 45 03    STA $0345      - A:04 X:05 Y:80 SP:f2 ..-.....   10476866

(C:$c01e) 

.C:c021  E0 08       CPX #$08       - A:04 X:05 Y:80 SP:f2 ..-.....   10476870

(C:$c021) 

.C:c023  D0 E8       BNE $C00D      - A:04 X:05 Y:80 SP:f2 N.-.....   10476872

(C:$c023) 

.C:c00d  AD 3C 03    LDA $033C      - A:04 X:05 Y:80 SP:f2 N.-.....   10476875

(C:$c00d) 

.C:c010  2D 45 03    AND $0345      - A:55 X:05 Y:80 SP:f2 ..-.....   10476879

(C:$c010) X counter is 5, A is our convert value, TESTYBYE is stored as 00000100

.C:c013  C9 00       CMP #$00       - A:04 X:05 Y:80 SP:f2 ..-.....   10476883

(C:$c013) 

.C:c015  D0 17       BNE $C02E      - A:04 X:05 Y:80 SP:f2 ..-....C   10476885

(C:$c015) 

.C:c02e  A9 31       LDA #$31       - A:04 X:05 Y:80 SP:f2 ..-....C   10476888

(C:$c02e) 

.C:c030  9D 08 07    STA $0708,X    - A:31 X:05 Y:80 SP:f2 ..-....C   10476890

(C:$c030) 

.C:c033  4C 19 C0    JMP $C019      - A:31 X:05 Y:80 SP:f2 ..-....C   10476895

(C:$c033) 

.C:c019  E8          INX            - A:31 X:05 Y:80 SP:f2 ..-....C   10476898
(C:$c019) Make X, or cycle counter 6, X=X+1
.C:c01a  AD 45 03    LDA $0345      - A:31 X:06 Y:80 SP:f2 ..-....C   10476900

(C:$c01a) 

.C:c01d  4A          LSR A          - A:04 X:06 Y:80 SP:f2 ..-....C   10476904

(C:$c01d) 

.C:c01e  8D 45 03    STA $0345      - A:02 X:06 Y:80 SP:f2 ..-.....   10476906

(C:$c01e) 

.C:c021  E0 08       CPX #$08       - A:02 X:06 Y:80 SP:f2 ..-.....   10476910

(C:$c021) 

.C:c023  D0 E8       BNE $C00D      - A:02 X:06 Y:80 SP:f2 N.-.....   10476912

(C:$c023) 

.C:c00d  AD 3C 03    LDA $033C      - A:02 X:06 Y:80 SP:f2 N.-.....   10476915

(C:$c00d) 

.C:c010  2D 45 03    AND $0345      - A:55 X:06 Y:80 SP:f2 ..-.....   10476919

(C:$c010) X counter is 6, A is our convert value, TESTYBYE is stored as 00000010

.C:c013  C9 00       CMP #$00       - A:00 X:06 Y:80 SP:f2 ..-...Z.   10476923

(C:$c013) 

.C:c015  D0 17       BNE $C02E      - A:00 X:06 Y:80 SP:f2 ..-...ZC   10476925

(C:$c015) 

.C:c017  F0 0D       BEQ $C026      - A:00 X:06 Y:80 SP:f2 ..-...ZC   10476927

(C:$c017) 

.C:c026  A9 30       LDA #$30       - A:00 X:06 Y:80 SP:f2 ..-...ZC   10476930

(C:$c026) 

.C:c028  9D 08 07    STA $0708,X    - A:30 X:06 Y:80 SP:f2 ..-....C   10476932

(C:$c028) 

.C:c02b  4C 19 C0    JMP $C019      - A:30 X:06 Y:80 SP:f2 ..-....C   10476937

(C:$c02b) 

.C:c019  E8          INX            - A:30 X:06 Y:80 SP:f2 ..-....C   10476940
(C:$c019) Make X, or cycle counter 7, X=X+1
.C:c01a  AD 45 03    LDA $0345      - A:30 X:07 Y:80 SP:f2 ..-....C   10476942

(C:$c01a) 

.C:c01d  4A          LSR A          - A:02 X:07 Y:80 SP:f2 ..-....C   10476946

(C:$c01d) 

.C:c01e  8D 45 03    STA $0345      - A:01 X:07 Y:80 SP:f2 ..-.....   10476948

(C:$c01e) 

.C:c021  E0 08       CPX #$08       - A:01 X:07 Y:80 SP:f2 ..-.....   10476952

(C:$c021) 

.C:c023  D0 E8       BNE $C00D      - A:01 X:07 Y:80 SP:f2 N.-.....   10476954

(C:$c023) 

.C:c00d  AD 3C 03    LDA $033C      - A:01 X:07 Y:80 SP:f2 N.-.....   10476957

(C:$c00d) 

.C:c010  2D 45 03    AND $0345      - A:55 X:07 Y:80 SP:f2 ..-.....   10476961

(C:$c010) X counter is 7, A is our convert value, TESTYBYE is stored as 00000001

.C:c013  C9 00       CMP #$00       - A:01 X:07 Y:80 SP:f2 ..-.....   10476965

(C:$c013) 

.C:c015  D0 17       BNE $C02E      - A:01 X:07 Y:80 SP:f2 ..-....C   10476967

(C:$c015) 

.C:c02e  A9 31       LDA #$31       - A:01 X:07 Y:80 SP:f2 ..-....C   10476970

(C:$c02e) 

.C:c030  9D 08 07    STA $0708,X    - A:31 X:07 Y:80 SP:f2 ..-....C   10476972

(C:$c030) 

.C:c033  4C 19 C0    JMP $C019      - A:31 X:07 Y:80 SP:f2 ..-....C   10476977

(C:$c033) 

.C:c019  E8          INX            - A:31 X:07 Y:80 SP:f2 ..-....C   10476980
(C:$c019) Make X, or cycle counter 8, X=X+1
.C:c01a  AD 45 03    LDA $0345      - A:31 X:08 Y:80 SP:f2 ..-....C   10476982

(C:$c01a) 

.C:c01d  4A          LSR A          - A:01 X:08 Y:80 SP:f2 ..-....C   10476986

(C:$c01d) 

.C:c01e  8D 45 03    STA $0345      - A:00 X:08 Y:80 SP:f2 ..-...ZC   10476988

(C:$c01e) 

.C:c021  E0 08       CPX #$08       - A:00 X:08 Y:80 SP:f2 ..-...ZC   10476992
(C:$c021) X is equal to 8, we no longer loop
.C:c023  D0 E8       BNE $C00D      - A:00 X:08 Y:80 SP:f2 ..-...ZC   10476994

(C:$c023) End of program, return to BASIC

.C:c025  60          RTS            - A:00 X:08 Y:80 SP:f2 ..-...ZC   10476996
(C:$c025) 

Heres another thing to do thats really cool

Go to Virtual 6502 and paste the program in. Here is is compiled

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

Set the start address hex to C000

Click LOAD MEMORY then
Click SHOW MEMORY

Click the Green PC on the top left and type in C000, click OK

Click Continuous run, and watch the program execute, total cycles should be 356.  Click Look up me # and type in 0708.  you will see the screen codes for 01010101, a combination of 30 and 31

NEXT----->
Understanding 6502 assembly on the Commodore 64 - (14) Space and cycle optimization


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