From mboxrd@z Thu Jan  1 00:00:00 1970
From: will.senn@gmail.com (Will Senn)
Date: Sat, 26 Dec 2015 12:34:15 -0600
Subject: [TUHS] v6 RK05 bootloader question
Message-ID: <567EDDA7.1000104@gmail.com>

All,

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:

RK05

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 
decremented R0(RKWC)
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

Lines 1-2
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.

Line 3
The RKDA buffer is cleared, setting the disk address to 0.

Line 4
The RKBA buffer is cleared, setting the bus address to 0.

Line 5
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.

Lines 6-7
The value 5 is placed into RKCS, this value represents a read operation 
and go.

Lines 8-9
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.

Line 10
  PC is set to 00000 and execution of the bytes read from the disk 
begins at location 00000.