http://altairbasic.org/

Altair BASIC 3.2 (4K) - Annotated Disassembly

[This is a work in progress, about 90% complete]

Here is a complete annotated disassembly of Altair BASIC 3.2 (4K). I
have faithfully followed the program order, and the code naturally
divides into three sections - the Interpreter, the Maths Package, and
Initialisation.

Each of these sections is more-or-less logically subdivided into a
number of subsections.

 

1. The Interpreter

Interpreter (Explanation) - Explanations and overviews of most of the
internal logic used in BASIC. It describes the memory layout, key
concepts, structures and variables, and you shouldn't look at a line
of code until you've read through this.

     Section        Base Size Key Functions
1.1  Restarts       0000 61   Start, SyntaxCheck, NextChar, OutChar,
                              CompareHLDE, FTestSign, PushNextWord
1.2  Keyword Tables 003D 189  [no code in this section]

1.3  Error Codes &  00FA 152  [no code in this section]
     Globals
                              GetFlowPtr, CopyMemoryUp,
1.4  Utility        0192 101  CheckEnoughVarSpace, CheckEnoughMem,
     Functions                OutOfMemory, SyntaxError, DivideByZero,
                              Error
     The BASIC                Stop, Main, StoreProgramLine,
1.5  Prompt &       01F7 375  FindProgramLine, New, Run, ResetAll,
     Program                  InputLineWith'?', Tokenize, InputLine
     Storage
1.6  Terminal I/O   036E 32   OutChar_tail, InputChar
1.7  LIST Handler   038E 71   List
1.8  FOR Handler    03D5 76   For
1.9  Execution      0421 61   ExecNext, Exec
1.10 More Utility   045E 63   NextChar_tail, Restore, TestBreakKey,
     Functions                CharIsAlpha, GetSubscript
1.11  Goto, Gosub,  049D 101  LineNumberFromStr, Gosub, Goto, Return,
     and Return               FindNextStatement
1.12 Assigning      0502 20   Let, AssignVar
     Variables
1.13 IF Handler     0516 63   If
1.14 Printing       0555 143  Print, NewLine, PrintString,
                              ToNextTabBreak, Tab
1.15 INPUT & READ   05E4 101  Input, Read
     Handlers
1.16 NEXT Handler   0649 65   Next
1.17 Expression     068A 135  EvalExpression, EvalTerm
     Evaluation
1.18 Variable       0711 232  Dim, GetVar, GetArrayVar
     Management

Total size: 2041 bytes

---------------------------------------------------------------------

 

2. The Math Package

Math package (explained) - All the concepts needed to understand the
math package.


     Section        Base Size Key Functions
2.1  Utility        07F9 17   FWordToFloat, FAddOneHalf
     Functions
     Addition &
2.2  Subtraction    080A 81   FSub, FAdd

                              FNormalise, FMantissaLeft,
2.3  Mantissa Magic 085B 136  FMantissaInc, Overflow, FAddMantissas,
                              FNegateInt, FMantissaRtOnce,
                              FMantissaRtMult
2.4  Multiplication 08E3 247  FMul, FDiv, FDivByTen, FMulByTen,
     & Division               FZero, FExponentAdd
                              FTestSign_tail, InvSignToInt,
2.5  Sign Magic     09DA 40   SignToInt, Sgn, FCharToFloat, Abs,
                              FNegate
2.6  Moving FACCUM  0A02 53   FPush, FLoadFromMem, FLoadFromBCDE,
     about                    FCopyToBCDE, FLoadBCDE, FCopyToMem
2.7  Unpacking &    0A37 64   FUnpack, FCompare
     Comparison
2.8  Converting to  0A77 60   FAsInteger, FMantissaDec, Int
     Integers
2.9  Reading        0AB3 124  FIn
     Numbers
2.10 Printing       0B2F 242  PrintIN, PrintInt, FOut
     Numbers
2.11 SQR            0C21 62   Sqr
2.12 RND            0C5F 54   Rnd
2.13 SIN            0C95 130  Sin

Total size: 1307 bytes.

---------------------------------------------------------------------

 

3. Initialisation

      Section                 Base       Size Key Functions
3.1   IO Detection            0D21    109     Init
3.2   Configuration           0D8E    367      

Total size: 478 bytes.