PDP-11 Bootstrap Loader - Revision history

Jdl: Slightly clearer description of xx

2025-03-05T04:48:40Z

Slightly clearer description of xx

Jdl: Transfer byte to buffer, not word

2025-03-05T04:39:36Z

Transfer *byte* to buffer, not word

Jdl: Replaced bootstrap code with valid macro11 assembly that more closely matches the DEC documentation. Replaced "0x" placeholder text with "xx" to avoid confusion with C hex prefix. Added tables of possible xx and yyyyyy values.

2025-03-05T04:36:47Z

Replaced bootstrap code with valid macro11 assembly that more closely matches the DEC documentation. Replaced "0x" placeholder text with "xx" to avoid confusion with C hex prefix. Added tables of possible xx and yyyyyy values.

Jnc: Better cat

2023-11-16T17:52:20Z

Better cat

Jnc: +cat

2022-04-05T21:02:15Z

+cat

Jnc: /* Code */ Offset in symbolic form

2021-03-08T09:27:31Z

‎Code: Offset in symbolic form

Jnc: /* Bootstrap Loader format */ correc detail

2019-08-20T14:50:45Z

‎Bootstrap Loader format: correc detail

Jnc: /* Program */ correct loc

2019-08-20T14:47:34Z

‎Program: correct loc

Jnc: /* Program */ add actual code

2019-08-20T01:12:31Z

‎Program: add actual code

Jnc: Mention LSI-11 ODT

2019-08-19T18:47:19Z

Mention LSI-11 ODT

@@ Line 38: / Line 38: @@
 </pre>
-When inputting the bootstrap program, the first word is loaded at location xx7744, where xx is replaced with the
+When inputting the bootstrap program, the first instruction should be placed at location xx7744,
-address of the highest memory bank available according to the following table:
+where xx is replaced with the address of the highest memory bank available according to the following table:
 {| class="wikitable" style="text-align:center;"

@@ Line 29: / Line 29: @@
 xx7756  105711  WAIT:   TSTB @R1             ; Wait until data available
 xx7760  100376          BPL WAIT
-xx7762  116162          MOVB 2(R1), LOAD(R2) ; Transfer word to buffer
+xx7762  116162          MOVB 2(R1), LOAD(R2) ; Transfer byte to buffer
 xx7764  000002
 xx7766  xx7400

@@ Line 16: / Line 16: @@
 This is the actual bootstrap program:
- 	LOAD = 0x7400				; Buffer address
+<pre>
-x7744	016701	BEGIN:	MOV	DEVICE, R1	; Get Device CSR
+                        LOAD=xx7400          ; Buffer start address
-x7746	000026
-x7750	012702	LOOP:	MOV	(PC)+, R2	; Get buffer offset
+                        .=LOAD+0344          ; Start address of bootstrap loader (xx7744)
-x7752	000352	OFFSET:	.-LOAD
-x7754	005211		INC	@R1		; Turn on reader
+xx7744  016701  START:  MOV DEVICE, R1       ; Get reader CSR address
-x7756	105711	READY:	TSTB	@R1		; Done?
+xx7746  000026
-x7760	100376		BPL	READY
+xx7750  012702  LOOP:   MOV #.-LOAD+2, R2    ; Get buffer pointer
-x7762	116162		MOVB	2(R1), LOAD(R2)	; Transfer
+xx7752  000352                               ;   (<--- pointer to buffer)
-x7764	000002
+xx7754  005211          INC @R1              ; Enable the paper tape reader
-x7766	0x7400
+xx7756  105711  WAIT:   TSTB @R1             ; Wait until data available
-x7770	005267		INC	OFFSET		; Bump buffer offset
+xx7760  100376          BPL WAIT
-x7772	177756
+xx7762  116162          MOVB 2(R1), LOAD(R2) ; Transfer word to buffer
-x7774	000765		BR	LOOP
+xx7764  000002
-x7776	yyyyyy	DEVICE:	yyyyyy			; Input device CSR address
+xx7766  xx7400
 ==Bootstrap Loader format==

@@ Line 54: / Line 54: @@
 The Bootstrap Loader is documented in some detail in the "paper tape software programming handbook" (DEC-11-GGPB-D), pp. 6-1 to 6-8.
-[[Category: PDP-11s]]
+[[Category: PDP-11 Software]]
 [[Category: PDP-11 File Formats]]

@@ Line 1: / Line 1: @@
-The '''PDP-11 Bootstrap Loader''' is a short [[program]] designed to be loaded into the [[main memory]] on a [[PDP-11]] via the [[front panel]]; it is then run to load an initial program into the machine. I.e it is a way of [[bootstrap]]ping a totally empty machine, one with no [[Read-only memory|ROM]].
+The '''PDP-11 Bootstrap Loader''' is a short [[program]] designed to be loaded into the [[main memory]] on a [[PDP-11]] via the [[front panel]]; it is then run to load an initial program into the machine. I.e. it is a way of [[bootstrap]]ping a totally empty machine, one with no [[Read-only memory|ROM]].
 It is designed to be absolutely as short as possible (since it is manually toggled in via the [[switch register]]), and therefore has limitations, including how long a program can be loaded using it, and where in memory that program can be placed. The usual program to load with it is the [[PDP-11 Absolute Loader]], which relaxes both of those limitations.
@@ Line 11: / Line 11: @@
 The program starts with the pointer set to itself, so the blank leader at the start of the data for the bootstrap loader (originally contained on [[paper tape]]s) starts with a special leader code which causes this to be left un-changed.
-The buffer pointer is then modified to point it slightly before the bootrap loader in memory, so at the end of the load the newly loaded code then progressively overlays the first several [[instruction]]s of the bootstrap loader itself, finally patching the buffer pointer to point at the branch instruction, which is then modified to jump to the newly loaded code.
+The buffer pointer is then modified to point it slightly before the bootstrap loader in memory, so at the end of the load the newly loaded code then progressively overlays the first several [[instruction]]s of the bootstrap loader itself, finally patching the buffer pointer to point at the branch instruction, which is then modified to jump to the newly loaded code.
 ===Code===
@@ Line 53: / Line 53: @@
 The Bootstrap Loader is documented in some detail in the "paper tape software programming handbook" (DEC-11-GGPB-D), pp. 6-1 to 6-8.
 [[Category: PDP-11s]]

@@ Line 40: / Line 40: @@
 * Load offset (1 byte)
 * Program to be loaded (up to 0344 bytes)
-* Copy of boostrap loader (first 7 bytes)
+* Copy of boostrap loader (first 6 bytes)
-* Branch offset (1 byte)
+* Offset to branch (1 byte)
 ==Others==

@@ Line 5: / Line 5: @@
 The original bootstrap loader is [[self-modifying code]], since this allows a very short program. The program is not directly self-modifying; rather, the nature of its input is such that it modifies the bootstrap loader as it is read in. (Most bootstrap loader input files eventually restore the bootstrap loader to its original condition, allowing it to be re-used.)
-==Structure==
+==Program==
 The bootstrap loader contains two locations which are modified by reading the input: the first is a [[pointer]] to the [[buffer]] in which to place data being read in; the second is a [[branch]] back to the beginning of the data input [[loop]].
@@ Line 12: / Line 12: @@
 The buffer pointer is then modified to point it immediately before the bootrap loader in memory, so at the end of the load the newly loaded code then progressively overlays the first several [[instruction]]s of the bootstrap loader itself, finally patching the buffer pointer to point at the branch instruction, which is then modified to jump to the newly loaded code.
 ==Bootstrap Loader format==