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REV.8
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1987-04-07
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4KB
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87 lines
;---------------
; REV
;---------------
; REV is a filter that reverses all lines of standard input, and sends
; the reversed lines to standard output.
DATA SEGMENT
ORG 02000 ; small model-- start data well beyond this program
OBUFF: ; output buffer
DB 02000 DUP (?)
OBUFF_LIM: ; limit signalling we are near the end of the output buffer
DB 0100 DUP (?)
DATA ENDS
REV:
MOV BP,OBUFF ; initialize the output buffer to empty
CALL VERIFY_NO_ARGS ; verify that there were no arugments in the command tail
CALL PROCESS_INPUT ; reverse all the line of standard input
CALL FLUSH_OUTPUT ; flush the output buffer
JMP GOOD_EXIT ; success-- exit back to the operating system
; VERIFY_NO_ARGS insures that there are no command-tail arguments at the
; PSP buffer DS:080. If there are, we abort with USAGE_MSG.
VERIFY_NO_ARGS:
MOV SI,080 ; point to the PSP command-tail
LODSB ; fetch the length byte
CBW ; extend length AL into AX
XCHG CX,AX ; swap the command tail length into CX
JCXZ RET ; good return if there is no tail whatsoever
L1: ; loop here for each byte of the command tail
LODSB ; fetch a command-tail byte
CMP AL,' ' ; it had better be a blank or a control character
IF A JMP USAGE_EXIT ; error exit if command tail has non-blank contents
LOOP L1 ; loop for the next command-tail byte
RET
USAGE_MSG:
DB 'usage: rev <in >out',0D,0A
USAGE_SIZE EQU $ - USAGE_MSG
; PROCESS_LINE reverses the line of CX bytes pointed to by SI (and beyond by
; DI), and copies the reversed line to the output buffer.
PROCESS_LINE:
DEC DI ; decrement beyond-pointer back to linefeed
CMP B[DI-1],0D ; is there also a CR terminating this line?
IF E DEC DI ; if yes then decrment back to the CR, also
CALL REV_STRING ; reverse the characters of the line
MOV DI,BP ; fetch the output buffer pointer
REP MOVSB ; copy the reversed line to the output buffer
MOV BP,DI ; store the updated output buffer pointer
CMP BP,OBUFF_LIM ; is the output buffer almost full?
JB RET ; return if not
FLUSH_OUTPUT:
PUSH AX,BX,CX,DX ; preserve registers across call
MOV DX,OBUFF ; point to the start of the output buffer
MOV CX,BP ; point CX beyond the bytes we have placed in buffer
SUB CX,DX ; calculate the number of bytes in the buffer
MOV BP,DX ; reset the output pointer back to the buffer start
MOV BX,1 ; file handle for standard output is 1
CALL MWRITE ; write the buffered bytes to standard output
POP DX,CX,BX,AX ; restore clobbered registers
RET
; REV_STRING reverses the string running from DS:SI through DS:DI-1. We do so
; by repeatedly exchanging the bytes pointed to by each pointer, then moving
; the pointers in towards each other, until they meet.
REV_STRING:
CMP SI,DI
JE RET
PUSH SI,DI ; save registers across call
L1: ; loop here for each pair of bytes
LODSB ; fetch the SI-pointed byte
DEC DI ; decrement DI to the last byte of the remaining string
XCHG AL,[DI] ; exchange the SI-pointed byte with the DI-pointed byte
MOV [SI-1],AL ; store the DI-pointed byte in the SI-pointed spot
CMP SI,DI ; have SI and DI come together yet?
JB L1 ; loop if not to swap two more end-bytes
POP DI,SI ; restore clobbered registers
RET ; NoCarry set for disassembler MEMDIS_n's benefit