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2A52: THE 'SLICING' SUBROUTINE
Used by the routines at S_LETTER and STK_VAR.
The present string can be sliced using this subroutine. The subroutine is entered with the parameters of the string being present on the top of the calculator stack and in the registers A, B, C, D and E. Initially the SYNTAX/RUN flag is tested and the parameters of the string are fetched only if a line is being executed.
Input
BC Length of the string
DE Address of the first character in the string
Output
SLICING 2A52 CALL SYNTAX_Z Check the flag.
2A55 CALL NZ,STK_FETCH Take the parameters off the stack in 'run-time'.
The possibility of the 'slice' being '()' has to be considered.
2A58 RST $20 Get the next character.
2A59 CP ")" Is it a ')'?
2A5B JR Z,SL_STORE Jump forward if it is so.
Before proceeding the registers are manipulated as follows:
2A5D PUSH DE The 'start' goes on the machine stack.
2A5E XOR A The A register is cleared and saved.
2A5F PUSH AF
2A60 PUSH BC The 'length' is saved briefly.
2A61 LD DE,$0001 Presume that the 'slice' is to begin with the first character.
2A64 RST $18 Get the first character.
2A65 POP HL Pass the 'length' to HL.
The first parameter of the 'slice' is now evaluated.
2A66 CP $CC Is the present character a 'TO'?
2A68 JR Z,SL_SECOND The first parameter, by default, will be '1' if the jump is taken.
2A6A POP AF At this stage A is zero.
2A6B CALL INT_EXP2 BC is made to hold the first parameter. A will hold +FF if there has been an 'out of range' error.
2A6E PUSH AF Save the value anyway.
2A6F LD D,B Transfer the first parameter to DE.
2A70 LD E,C
2A71 PUSH HL Save the 'length' briefly.
2A72 RST $18 Get the present character.
2A73 POP HL Restore the 'length'.
2A74 CP $CC Is the present character a 'TO'?
2A76 JR Z,SL_SECOND Jump forward to consider the second parameter if it is so; otherwise show that there is a closing bracket.
2A78 CP ")"
This entry point is used by the routine at STK_VAR.
SL_RPT_C 2A7A JP NZ,REPORT_C
At this point a 'slice' of a single character has been identified. e.g. A$(4).
2A7D LD H,D The last character of the 'slice' is also the first character.
2A7E LD L,E
2A7F JR SL_DEFINE Jump forward.
The second parameter of a 'slice' is now evaluated.
SL_SECOND 2A81 PUSH HL Save the 'length' briefly.
2A82 RST $20 Get the next character.
2A83 POP HL Restore the 'length'.
2A84 CP ")" Is the present character a ')'?
2A86 JR Z,SL_DEFINE Jump if there is not a second parameter.
2A88 POP AF If the first parameter was in range A will hold zero, otherwise +FF.
2A89 CALL INT_EXP2 Make BC hold the second parameter.
2A8C PUSH AF Save the 'error register'.
2A8D RST $18 Get the present character.
2A8E LD H,B Pass the result obtained from INT_EXP2 to the HL register pair.
2A8F LD L,C
2A90 CP ")" Check that there is a closing bracket now.
2A92 JR NZ,SL_RPT_C
The 'new' parameters are now defined.
SL_DEFINE 2A94 POP AF Fetch the 'error register'.
2A95 EX (SP),HL The second parameter goes on the stack and the 'start' goes to HL.
2A96 ADD HL,DE The first parameter is added to the 'start'.
2A97 DEC HL Go back a location to get it correct.
2A98 EX (SP),HL The 'new start' goes on the stack and the second parameter goes to HL.
2A99 AND A Subtract the first parameters from the second to find the length of the 'slice'.
2A9A SBC HL,DE
2A9C LD BC,$0000 Initialise the 'new length'.
2A9F JR C,SL_OVER A negative 'slice' is a 'null string' rather than an error condition.
2AA1 INC HL Allow for the inclusive byte.
2AA2 AND A Only now test the 'error register'.
2AA3 JP M,REPORT_3 Jump if either parameter was out of range for the string.
2AA6 LD B,H Transfer the 'new length' to BC.
2AA7 LD C,L
SL_OVER 2AA8 POP DE Get the 'new start'.
2AA9 RES 6,(IY+$01) Ensure that a string is still indicated (reset bit 6 of FLAGS).
SL_STORE 2AAD CALL SYNTAX_Z Return at this point if checking syntax; otherwise continue into STK_ST_0.
2AB0 RET Z
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