Radix 100 notations
RAM addresses
Floating point routines
In the TI-99/4A real numbers are represented in radix 100, floating point notation. This notation uses 8 bytes per number. It is different from that used on PCs and yet extremely efficient: at equal processing power a TI-99/4A would do much better than a PC.
The exponent is 1 byte long. Its value is biased by >40:
>40 is E+0
>41 is E+1
>3F is E-1
etc.
The mantissa is seven bytes long. Each byte contains a number between 0 and 99 (>00 and >63), providing up to 14 significant digits. The decimal point is assumed to be after the first mantissa byte (i.e. after the second digit).
Both the exponent and the first mantissa byte are negated, i.e. the first word is negated.
The number zero is an exception: the exponent and the first mantissa byte are both zero (i.e. the first word is >0000). The remaining bytes can have any value.
Exponent Mantissa Value
>40 >01 >02 >03 >04 >05 >06 >07 1.020304050607
>41 >01 >02 >03 >04 >05 >06 >0B 10.20304050611
>3F >05 >00 >00 >00 >00 >00 >00 0.5
>BF(-41) >FF >02 >03 >05 >05 >06 >12 -10.20304050618
>00 >00 >xx >xx >xx >xx >xx >xx 0 (any exponent!)
Most ROM and GROM routines that deal with real numbers use the following addresses in the scratch-pad RAM.
FAC: an 8 bytes area, from >834A to >8351 (Floating
point
ACcumulator)
ARG: an 8 bytes area, from >835C to >8363 (ARGument
accumulator)
SGN: byte >8375, sign of FAC (00 = positive, >FF =
negative)
EE: byte >8376, copy of FAC exponent
ERCODE: byte >8354, error code returned by floating point
calculation
routines.
VSPTR: bytes >836E->836F, pointer to a data stack in VDP
memory.
In Basic it is called the value stack, but you can use it even if your
program does not run with Basic: just put a relevant value in >836E-F
The following error codes can be found at >8354, after a real number operation. In general, the Cnd bit of the GPL status byte (>837C) is set, if an error occured.
>01 Overflow (warning) / Division by 0 (error)
>02 Syntax error
>03 Integer overflow, on conversion
>04 Square root of a negative number
>05 Negative number raised to non-integer power
>06 Log(0) or log of a negative number
>07 Trigonometric function error
A number of assembly and GPL routines that perform advanced calculations with floating points are at your disposal. In general, they should be called from GPL: the assembly routines as XML, the GPL routines with CALL. However, you can also use them in assembly, with a suitable calling routine.
You'll see in the table below that many floating point operations are performed by two distinct routines: one uses two numbers in the scratch-pad (in FAC and in ARG), the other a number in FAC and a number in the VDP stack. The stack pointer (in >836E-F) is automatically decremented by 8 to point at the next number once the operation is completed.
| Mnemonic | XML | Use |
|---|---|---|
| ROUND1 | >01 | Rounds number in FAC from 9 to 8 bytes. |
| ROUND | >02 | Rounds this number. Number of decimals in >835C. |
| STST | >03 | Test number in FAC and saves status. |
| OVEXP | >04 | Overflow / underflow. |
| OV | >05 | Overflow (part of the above procedure). |
| FADD | >06 | Adds two real numbers: FAC = FAC + ARG |
| FSUB | >07 | Substracts a real from another: FAC = ARG - FAC |
| FMUL | >08 | Multiplies two reals: FAC = FAC * ARG |
| FDIV | >09 | Divides real by another: FAC = FAC / ARG |
| FCOMP | >0A | Compares two reals (FAC with ARG) |
| SADD | >0B | Addition with stack: FAC = FAC + real_on_stack |
| SSUB | >0C | Substraction with stack: FAC = real_on_stack - FAC |
| SMUL | >0D | Multiplication with stack: FAC = FAC * real_on_stack |
| SDIV | >0E | Division with stack: FAC = FAC / real_on_stack |
| SCOMP | >0F | Comparison with stack: FAC with real_on_stack |
| CSN | >10 | Converts a string (ptr in >8356) to a real (in FAC), from Basic |
| CSNGR | >11 | Same as above, entry point from GPL. |
| CFI | >12 | Converts real to integer (from FAC to FAC) |
| VPUSH | >17 | Pushes a value on the VDP stack, from FAC |
| VPOP | >18 | Pops a value from the VDP stack to FAC |
| Mnemonic | Address | Use |
|---|---|---|
| CNS | >0014 | Convert number to string (from FAC to FAC, infos in >8355-57) |
| INT | >0022 | Convert real to integer (from FAC to FAC) |
| PWR | >0024 | Power-of-ten routine: FAC=FAC * 10^ARG |
| SQR | >0026 | Square root routine: FAC=SQR(FAC) |
| EXP | >0028 | Exponentiation routine: FAC=e^FAC |
| LOG | >002A | Logarithm calculation: FAC=ln(FAC) |
| COS | >002C | Cosine calculation: FAC=cos(FAC) |
| SIN | >002E | Sine calculation: FAC=sin(FAC) |
| TAN | >0030 | Tangent calculation: FAC=tan(FAC) |
| ATN | >0032 | Arctangent calculation: FAC=atn(FAC) |