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9EBD '-' operator - Subtraction

Submitted by Steve Fewell

Description:

Check the value type (in A).
If the value is a string then issue a Type Mismatch error, as you cannot subtract
Strings in BASIC.

If variable type is negative then the first value is a Float, so jump to &9EE7, which
does the following:
* Push FWA to Stack
* Get the second value from the expression handler (level 5)
* If second value is a String then generate a Type Mismatch error.
* If second value is an Integer then convert it to a Float
* Pop the first Float value from Stack to argp (&4A, &4B)
* Call the Floating-Point Subtraction routine to subtract the 2nd value from the 1st (&A68A)
* Jump to 9E4F (Expression handler level 4) to check for further + or - operators.

Otherwise, the value is Integer, so push the first value to the Stack and call the
Expression Handler level 5 routine (&9FC4) to get the second value to add.
If the second value is a string then generate a Type Mismatch error.

If the second value is a Float then jump to &9EFF to do the following:
* Pop the first value (the Integer) from the Stack
* Push the second value (the Float) to the Stack
* Convert the Integer to a Float (and put it in the FWA).
* Pop the second Float value from Stack to argp (&4A, &4B)
* Call the Floating-Point Subtraction routine to subtract the 1st value from the 2nd (&ACC7)
* If the FWA is not zero, then reverse the sign of the FWA (as the subtraction was performed
  the opposite way than it should have been.
* Jump to 9E4F (Expression handler level 4) to check for further + or - operators.

Otherwise, both values are Integers, so carry out the Integer Addition routine. This
routine automatically jumps back to 9E4F on completion, to check for further + or - operators.


Disassembly for the '-' Operator - Subtraction routine

9EBD   168 A8 TAY
9EBE   240 209 F0 D1 BEQ -47 --> &9E91 [JMP &9092 - Type Mismatch error]
9EC0 0% 048 037 30 25 BMI 37 --> &9EE7
9EC2   032 196 159 20 C4 9F JSR &9FC4 Push Integer to stack & Expression handler level 5
9EC5   168 A8 TAY
9EC6   240 201 F0 C9 BEQ -55 --> &9E91 [JMP &9092 - Type Mismatch error]
9EC8 05 048 053 30 35 BMI 53 --> &9EFF
9ECA iminus start:

Disassembly for the Floating Point '-' Operator

9EE7   032 250 187 20 FA BB JSR &BBFA Push FWA to Stack
9EEA   032 199 159 20 C7 9F JSR &9FC7 Expression Handler level 5 (*,/,MOD,DIV)
9EED   168 A8 TAY
9EEE   240 161 F0 A1 BEQ -95 --> &9E91 [JMP &9092 - Type Mismatch error]
9EF0 ' 134 039 86 27 STX &27
9EF2 0 048 003 30 03 BMI 3 --> &9EF7
9EF4   032 133 129 20 85 81 JSR &8185 Convert Integer to Float
9EF7   032 232 187 20 E8 BB JSR &BBE8 Pop Float to argp
9EFA   032 138 166 20 8A A6 JSR &A68A Floating-Point Subtraction
9EFD   128 171 80 AB BRA -85 --> &9EAA
9EFF ' 134 039 86 27 STX &27
9F01   032 230 188 20 E6 BC JSR &BCE6 Pop Integer from Stack
9F04   032 250 187 20 FA BB JSR &BBFA Push FWA to Stack
9F07   032 133 129 20 85 81 JSR &8185 Convert Integer to Float
9F0A   032 232 187 20 E8 BB JSR &BBE8 Pop Float to argp
9F0D   032 199 172 20 C7 AC JSR &ACC7 Floating-Point Subtraction with reverse sign of result
9F10   128 152 80 98 BRA -104 --> &9EAA

Disassembly for the Floating Point Subtraction (with reverse sign of result) routine

ACC7   032 138 166 20 8A A6 JSR &A68A Floating-Point Subtraction
ACCA 1 165 049 A5 31 LDA &31
ACCC   240 006 F0 06 BEQ 6 --> &ACD4
ACCE . 165 046 A5 2E LDA &2E
ACD0 I 073 128 49 80 EOR#&80
ACD2 . 133 046 85 2E STA &2E
ACD4   169 255 A9 FF LDA#&FF
ACD6 ` 096 60 RTS


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