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.