4Data Telesoftware Adaptor Issue 1.2: Routine at 185A

185A: check_page: Telesoftware page checksum.

Used by the routine at telesoftware_main.
check_page	185A	LD DE,(page_receive_buffer_pointer)	Set HL to page_receive_buffer_pointer + 40 (row 1, column 0).
	185E	LD HL,$0028
	1861	ADD HL,DE
	1862	LD BC,$0398	Set BC to 920 as loop counter.
check_parity_loop	1865	LD A,(HL)	Load next byte from receive buffer.
	1866	OR A	If even parity jump to return_error_1.
	1867	JP PE,return_error_1	If even parity jump to return_error_1.
	186A	INC HL	Loop through 920 characters (rows 1 to 23).
	186B	DEC BC
	186C	LD A,C
	186D	OR B
	186E	JR NZ,check_parity_loop
	1870	LD HL,$0028	Set HL to page_receive_buffer_pointer + 40 (row 1, column 0).
	1873	ADD HL,DE
	1874	LD BC,$0394	Set BC to 916 as loop counter and clear D (zero checksum).
	1877	LD D,$00	Set BC to 916 as loop counter and clear D (zero checksum).
checksum_loop	1879	LD A,(HL)	Load next byte from receive buffer.
	187A	XOR D	XOR with previous checksum.
	187B	LD D,A	XOR with previous checksum.
	187C	INC HL	Loop through 916 characters (rows 1 to 23 minus last four characters).
	187D	DEC BC
	187E	LD A,C
	187F	OR B
	1880	JR NZ,checksum_loop
	1882	PUSH DE	Preserve checksum.
	1883	EX DE,HL	Swap address of next byte into DE.
	1884	LD HL,checksum_nibbles	Set HL to address of checksum_nibbles.
	1887	CALL load_nibbles	Call load_nibbles.
	188A	POP DE	Restore checksum.
	188B	LD A,(checksum_nibbles)	Load low byte of checksum_nibbles.
	188E	XOR D	XOR with checksum and clear bit 7.
	188F	AND $7F	XOR with checksum and clear bit 7.
	1891	JP NZ,return_error_1	If any bits set jump to return_error_1.
	1894	LD A,($5B82)	If high byte of checksum_nibbles is not zero jump to return_error_2.
	1897	OR A
	1898	JP NZ,return_error_2
	189B	LD A,(blocks_remaining)	If blocks_remaining is not zero jump to check_page_0 else this is first block received.
	189E	OR A
	189F	JP NZ,check_page_0
	18A2	LD DE,(page_receive_buffer_pointer)	Set HL to page_receive_buffer_pointer + 925 (row 23, column 5) (number of blocks).
	18A6	LD HL,$039D
	18A9	ADD HL,DE
	18AA	LD A,(HL)	Copy low 6 bits to number_of_blocks and blocks_remaining.
	18AB	AND $3F
	18AD	LD (number_of_blocks),A
	18B0	LD (blocks_remaining),A
	18B3	LD HL,$03A4	Set HL to page_receive_buffer_pointer + 932 (row 23, column 12).
	18B6	ADD HL,DE
	18B7	LD DE,footer_buffer	Copy 24 characters to footer_buffer.
	18BA	LD BC,$0018
	18BD	LDIR
	18BF	JP return_ok	Skip forward to return success.
check_page_0	18C2	LD DE,(page_receive_buffer_pointer)	Set HL to page_receive_buffer_pointer + 925 (row 23, column 5) (number of blocks).
	18C6	LD HL,$039D
	18C9	ADD HL,DE
	18CA	LD A,(number_of_blocks)	Load number_of_blocks.
	18CD	XOR (HL)	Compare with byte at address in HL.
	18CE	AND $3F	Compare with byte at address in HL.
	18D0	JP NZ,return_error_2	If values don't match jump to return_error_2.
	18D3	LD HL,$03A4	Set HL to page_receive_buffer_pointer + 932 (row 23, column 12).
	18D6	ADD HL,DE
	18D7	LD DE,footer_buffer	Load address of footer_buffer into DE.
	18DA	LD B,$18	Set B to 24 as loop counter.
compare_bytes_loop	18DC	LD A,(DE)	Load next byte from footer_buffer into A.
	18DD	CP (HL)	Compare with byte in page receive buffer.
	18DE	JP NZ,return_error_2	If bytes don't match jump to return_error_2.
	18E1	INC HL	Loop through 24 bytes.
	18E2	INC DE
	18E3	DJNZ compare_bytes_loop
return_ok	18E5	XOR A	Return $00.
	18E6	RET	Return $00.
return_error_1	18E7	LD A,$01	Return error $01.
	18E9	RET	Return error $01.
return_error_2	18EA	LD A,$02	Return error $02.
	18EC	RET	Return error $02.