(2023-09-24) Create note

2023-09-24.md

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+# 2023-08-23 — Dev notes
+
+## Architecture changes
+
+### IO
+
+- need new status register, to hold current io device number
+    - could use the high byte of the Flags register, PSW style
+
+- need read/write instructions
+    - dev xx — select device xx
+        - or could do “ldd” and “std”
+    - inp xx — read one unit into memory, starting at xx
+    - out xx — write one byte from memory, starting at xx
+    - ? ota — write from A
+
+### 'B' register?
+
+- maybe? add B register, based on IBM 704/709/7090 “m/q”
+    - i thought this might be needed as an IO buffer but now I’m thinking not
+    - might still need/want it for mul/div tho
+    - would need new instructions
+        - ldb
+        - stb
+        - tab
+        - tba
+        - ? xba (exchange a and b)
+
+
+## System/360 IPL
+
+From the [Wikipedia article on booting](https://en.m.wikipedia.org/wiki/Booting):
+
+> In the System/360 processors, an IPL is initiated by the computer operator by selecting the three hexadecimal digit device address (CUU; C=I/O Channel address, UU=Control unit and Device address[nb 2]) followed by pressing the LOAD button.
+
+…
+
+> The IPL function in the System/360 and its successors prior to IBM Z, and its compatibles such as Amdahl's, reads 24 bytes from an operator-specified device into main storage starting at real address zero. The second and third groups of eight bytes are treated as Channel Command Words (CCWs) to continue loading the startup program (the first CCW is always simulated by the CPU and consists of a Read IPL command, 02h, with command chaining and suppress incorrect length indication being enforced). When the I/O channel commands are complete, the first group of eight bytes is then loaded into the processor's Program Status Word (PSW) and the startup program begins execution at the location designated by that PSW.
+
+…
+
+> The disk, tape or card deck must contain a special program to load the actual operating system or standalone utility into main storage, and for this specific purpose "IPL Text" is placed on the disk by the stand-alone DASDI (Direct Access Storage Device Initialization) program or an equivalent program running under an operating system, e.g., ICKDSF, but IPL-able tapes and card decks are usually distributed with this "IPL Text" already present.