I’m Benjamin Misell

@benjaminmisell

github.com/benjaminmisell

Tales of C, the 6502 and the BBC

Because python is awesome

With added python

I’m not

From the 80’s

​

It all starts with

With 80’s appropriate logo

The computer literacy project

The Acorn BBC Micro

6502 assembly

ADC....add with carry�AND....and (with accumulator)�ASL....arithmetic shift left�BCC....branch on carry clear�BCS....branch on carry set�BEQ....branch on equal (zero set)�BIT....bit test�BMI....branch on minus (negative set)�BNE....branch on not equal (zero clear)�BPL....branch on plus (negative clear)�BRK....interrupt�BVC....branch on overflow clear�BVS....branch on overflow set�CLC....clear carry�CLD....clear decimal�CLI....clear interrupt disable�CLV....clear overflow�

CMP .... compare (with accumulator)

CPX .... compare with X

CPY .... compare with Y

DEC .... decrement

DEX .... decrement X

DEY .... decrement Y

EOR .... exclusive or (with accumulator)

INC .... increment

INX .... increment X

INY .... increment Y

JMP .... jump

JSR .... jump subroutine

LDA .... load accumulator

LDY .... load X

LDY .... load Y

LSR .... logical shift right

NOP .... no operation

ORA .... or with accumulator

PHA .... push accumulator

PHP .... push processor status (SR)

PLA .... pull accumulator

​

PLP .... pull processor status (SR)

ROL .... rotate left

ROR .... rotate right

RTI .... return from interrupt

RTS .... return from subroutine

SBC .... subtract with carry

SEC .... set carry

SED .... set decimal

SEI .... set interrupt disable

STA .... store accumulator

STX .... store X

STY .... store Y

TAX .... transfer accumulator to X

TAY .... transfer accumulator to Y

TSX .... transfer stack pointer to X

TXA .... transfer X to accumulator

TXS .... transfer X to stack pointer

TYA .... transfer Y to accumulator

​

​

​

An if (passing)

LDA #$42 (Load accumulator)

CMP #$44 (Compare value in accumulator with absolute value)

BMI thing (Branch on minus)

JMP elsething

.thing *** Do thing ***

JMP endthing

.elsething *** Do other thing ***

.endthing

*** Rest of program ***

An if (failing)

LDA #$42 (Load accumulator)

CMP #$40 (Compare value in accumulator with absolute value)

BMI thing (Branch on minus)

JMP elsething

.thing *** Do thing ***

JMP endthing

.elsething *** Do other thing ***

.endthing

*** Rest of program ***

I can compilez

How does one compile?

What does a compiler do

Code

C in this case

Other code

Assembly in this case

What does a compiler do

Code

C in this case

Other code

Assembly in this case

Some things

Lexer, Parser, AST etc

Step 1

A lexer

A lexwhaaaat?

int main() {

return 1 + 2;

}

Token(INT, ‘int‘)

Token(ID, ‘main‘)

Token(LBRACK, ‘(‘)

Token(RBRACK, ‘)‘)

Token(LBRACE, ‘{‘)

Token(RETURN, ‘return‘)

Token(INT, 1)

Token(PLUS, ‘+‘)

Teken(INT, 2)

Token(SEMI, ‘;‘)

Token(RBRACE, ‘}‘)

Step 2

A parser and AST

Autonomous Sugary Topiary?

Token(INT, ‘int‘)

Token(ID, ‘main‘)

Token(LBRACK, ‘(‘)

Token(RBRACK, ‘)‘)

Token(LBRACE, ‘{‘)

Token(RETURN, ‘return‘)

Token(INT, 1)

Token(PLUS, ‘+‘)

Teken(INT, 2)

Token(SEMI, ‘;‘)

Token(RBRACE, ‘}‘)

​

TranslationUnit� Function<main:<IntegerCType(2:True)>>()� Compound� Return� Plus� Number<1>� Number<2>

Step 3

Assembly, finally

Compiled Assembly (No pun this time)

NEW

0 FOR opt%=0 TO 2 STEP 2

1 P%=&E00

2 [

3 OPT opt%

4 .__putchar

5 LDY #01

6 LDA (&8E),Y

7 JSR &FFEE

8 STA &71

9 LDA #00

10 STA &70

11 RTS

12

13 .__getchar

14 JSR &FFE0

15 STA &71

16 LDA #00

17 STA &70

18 RTS

19

20 .__osbyte

21 LDY #05

22 LDA (&8E),Y

​

​

23 TAY

24 LDY #03

25 LDA (&8E),Y

26 TAX

27 LDY #01

28 LDA (&8E),Y

29 JSR &FFF4

30 STX &71

31 LDA #00

32 STA &70

33 RTS

34

35 \ Routines

36 ._bbcc_pusha PHA

37 LDA &8E

38 BNE _bbcc_pusha_1

39 DEC &8F

40 ._bbcc_pusha_1

41 DEC &8E

42 PLA

43 LDY #00

44 STA (&8E),Y

45 RTS

46 ._bbcc_pulla LDY #0

​

47 LDA (&8E),Y

48 INC &8E

49 BEQ _bbcc_pulla_1

50 RTS

51 ._bbcc_pulla_1

52 INC &8F

53 RTS

54

55 \ Label

56 ._setup_global

57

58 \ Return

59 RTS

60

61 \ Function

62 ._start

63

64 \ Set

65 LDA #&00

66 STA &8E

67 LDA #&18

68 STA &8F

69

70 \ JmpSub

​

71 JSR _setup_global

72

73 \ CallFunction

74 JSR __main

75

76 \ Return

77 RTS

78

79 \ Function

80 .__main

81

82 \ Add

83 CLC

84 LDA #&01

85 ADC #&02

86 STA &71

87 LDA #&00

88 ADC #&00

89 STA &70

90

91 \ Return

92 RTS

93 ]

94 NEXT opt%

​

95 CALL _start

96 RH=&70

97 RL=&71

98 PRINT ~?RH,~?RL

​

Demo time

Or it be if I could SSH into a BBC Micro

Scrap that! Yes I can!

SSH into a what‽

One final thing

How do variables work?

A Symbol Table

A diagram (anyone spot a theme)

Diagram #I’ve lost count

Local Scope 2

Local Scope 1

Global Scope

Thank you

I’ve been Benjamin Misell

Questions?

@benjaminmisell

github.com/benjaminmisell/