[TUHS] v6 RK05 bootloader question
will.senn at gmail.com
Sun Dec 27 04:34:15 AEST 2015
I'm trying to understand the RK bootloader code that is found in
"Setting up Unix - Sixth Edition". My question is related to RKBA, RK's
bus address buffer. Is the bus address the same as memory address? If
so, the code makes sense, if not, I appreciate y'alls help.
Here's what I have so far:
01 012700 MOV 177414,R0 ; Move RKDB into R0
02 177414 ; RKDB Address
03 005040 CLR -(R0) ; Decrement R0 and clear the contents of RKDA
04 005040 CLR -(R0) ; Decrement R0 and clear the contents of RKBA
05 010040 MOV R0,-(R0) ; Move the contents of R0(RKBA) into
06 012740 MOV 5,-(R0) ; Decrement R0 and move 5 into RKCS
07 000005 ; Read and go
08 105710 WAIT: TSTB (R0) ; Test the lower byte of RKCS
09 002376 BGE WAIT ; When bit 7 becomes 1, the read is done
10 005007 CLR PC ; Set PC 000000, the start of the bytes read
RKDB - RK data buffer register
This register is a general data register and it only used by the code
above to initialize R0 so that subsequent RK addresses can be found by
simply decrementing R0.
RKDA - RK disk address register
This register determines the starting disk address of the read operation
and is cleared by the code.
RKBA - RK current bus address register
This register contains the bus address to or from which data will be
transferred. Is this the same as memory address?
RKWC - RK word count register
Two's complement of the number of words to be transferred.
RKCS - RK control status register
This is the register that controls the device and provides the device
status to the program
The execution of the boot loader code moves the address of RKDB into R0
to initialize the register so that it can be used to obtain the other RK
buffer addresses as they are needed.
The RKDA buffer is cleared, setting the disk address to 0.
The RKBA buffer is cleared, setting the bus address to 0.
The value in R0 is transferred into the RKWC buffer. RKBA or 177410, the
value in R0, is a convenient number to use for the read operation
because it is big enough to cause the program to read in a block of
data. The number is in two's complement and represents -370. This tells
the disk controller that 370 words (540 bytes) will be transferred.
The value 5 is placed into RKCS, this value represents a read operation
The lower byte of RKCS is tested and when it is greater than or equal to
zero (not negative), it loops, waiting until the value is negative, that
is until bit 7 becomes 1, which indicates Control Ready (RDY) and done.
PC is set to 00000 and execution of the bytes read from the disk
begins at location 00000.
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