5 ms·

My copy of The TTL Data Book suggests that the ALU was able to do XOR, an earlier blog post stated that microcode opcode bits go through a table instead of bein

by rjsw 9y ago

My copy of The TTL Data Book suggests that the ALU was able to do XOR, an earlier blog post stated that microcode opcode bits go through a table instead of being fed directly to the 74181 function pins. I wonder why Xerox didn't want to use all the ALU features.

kens 9y ago
In microcode, the Alto provides the 16 most useful functions of the 74181 (calling most of the 74181's operations "mostly useless"). This includes XOR and OR. However, the Alto copied the Data General Nova's instruction set, which doesn't include OR and XOR, so you can't use these instructions from machine code. I think there's extended microcode that provides "extra" instructions for XOR and OR and an improved BCPL compiler to make use of them, but I haven't tracked it down. See the Alto hardware manual (page 4) on bitsavers for details: http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/AltoHWRef.part1.pdf http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/...
- userbinator 9y ago
  Here is a document I found on the DG Nova instruction set: http://users.rcn.com/crfriend/museum/doco/DG/Nova/base-instr.html http://users.rcn.com/crfriend/museum/doco/DG/Nova/base-instr... If you scroll down to "Arithmetic/Logic Instructions" you'll see that they did not have room for XOR nor OR, since several of the 8 opcodes that fit into the 3-bit field are what we'd normally think of as "one-operand", but have been expanded to be "two-operand" (oddly enough, there is an increment but no decrement instruction either.) It's interesting to compare to two other well-known CPUs with a 3-bit ALU opcode field: The Z80's (and 8008/8080/8085) ALU opcodes are: ADD/ADC/SUB/SBC/AND/XOR/OR/CP The x86's ALU opcodes are: ADD/OR/ADC/SBB/AND/SUB/XOR/CMP
  - kens 9y ago
    That's an interesting comparison with microprocessor ALU operations. (6502 is similar to Intel but doesn't have add/subtract without carry.) Note that the Nova uses two additional instruction bits for the carry. Thus, the Intel instruction sets use two of the 8 opcodes for add with carry and subtract with borrow/carry, but the Nova doesn't. So it should be easier for the Nova to fit in additional useful ALU instructions. (Not to mention the Nova has 16-bit instructions.)
- rjsw 9y ago
  From your link there looks to be two functions spare, I guess you would need a new PROM to make use of them though. The MIT Lisp Machine does just map microcode instruction bits onto the 74181 function pins but it has much wider microinstructions than the 32-bit Alto ones.
- zkms 9y ago
  Is there documentation/examples for how to program the Alto's microcode?
  - kens 9y ago
    The Alto hardware manual explains how the microcode works (see sections 2 and 9 and try to memorize Figure 1). For examples, see the existing Alto microcode. The subsystem manual p90 describes the MU microcode assembler and the syntax. See also RPRAM in the subsystem manual that describes how to load microcode into control RAM. I haven't seen anything like a howto/tutorial for microcode in the Alto documentation. http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/AltoHWRef.part1.pdf http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/... http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/microcode/ http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/... http://www.mirrorservice.org/sites/www.bitsavers.org/pdf/xerox/alto/memos_1978/Alto_Subsystems_Apr78.pdf http://www.mirrorservice.org/sites/www.bitsavers.org/pdf/xer... If you want to try this stuff out for yourself (and don't have an Alto handy), use the ContrAlto emulator: https://github.com/livingcomputermuseum/ContrAlto https://github.com/livingcomputermuseum/ContrAlto
jecel 9y ago
An option is to use the BITBLT instruction which could be called from BCPL: ftp://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/BitBLT_Nov1975.pdf Dan Ingalls showed how to implement the Game of Life and how to rotate bitmaps using this instruction in a SIMD style. While BITBLT implemented all 16 possible boolean functions, not all were equally fast.

by rjsw 9y ago

kens 9y ago

In microcode, the Alto provides the 16 most useful functions of the 74181 (calling most of the 74181's operations "mostly useless"). This includes XOR and OR. However, the Alto copied the Data General Nova's instruction set, which doesn't include OR and XOR, so you can't use these instructions from machine code. I think there's extended microcode that provides "extra" instructions for XOR and OR and an improved BCPL compiler to make use of them, but I haven't tracked it down. See the Alto hardware manual (page 4) on bitsavers for details: http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/AltoHWRef.part1.pdf http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/...

userbinator 9y ago
Here is a document I found on the DG Nova instruction set: http://users.rcn.com/crfriend/museum/doco/DG/Nova/base-instr.html http://users.rcn.com/crfriend/museum/doco/DG/Nova/base-instr... If you scroll down to "Arithmetic/Logic Instructions" you'll see that they did not have room for XOR nor OR, since several of the 8 opcodes that fit into the 3-bit field are what we'd normally think of as "one-operand", but have been expanded to be "two-operand" (oddly enough, there is an increment but no decrement instruction either.) It's interesting to compare to two other well-known CPUs with a 3-bit ALU opcode field: The Z80's (and 8008/8080/8085) ALU opcodes are: ADD/ADC/SUB/SBC/AND/XOR/OR/CP The x86's ALU opcodes are: ADD/OR/ADC/SBB/AND/SUB/XOR/CMP
- kens 9y ago
  That's an interesting comparison with microprocessor ALU operations. (6502 is similar to Intel but doesn't have add/subtract without carry.) Note that the Nova uses two additional instruction bits for the carry. Thus, the Intel instruction sets use two of the 8 opcodes for add with carry and subtract with borrow/carry, but the Nova doesn't. So it should be easier for the Nova to fit in additional useful ALU instructions. (Not to mention the Nova has 16-bit instructions.)
rjsw 9y ago
From your link there looks to be two functions spare, I guess you would need a new PROM to make use of them though. The MIT Lisp Machine does just map microcode instruction bits onto the 74181 function pins but it has much wider microinstructions than the 32-bit Alto ones.
zkms 9y ago
Is there documentation/examples for how to program the Alto's microcode?
- kens 9y ago
  The Alto hardware manual explains how the microcode works (see sections 2 and 9 and try to memorize Figure 1). For examples, see the existing Alto microcode. The subsystem manual p90 describes the MU microcode assembler and the syntax. See also RPRAM in the subsystem manual that describes how to load microcode into control RAM. I haven't seen anything like a howto/tutorial for microcode in the Alto documentation. http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/AltoHWRef.part1.pdf http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/... http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/microcode/ http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/... http://www.mirrorservice.org/sites/www.bitsavers.org/pdf/xerox/alto/memos_1978/Alto_Subsystems_Apr78.pdf http://www.mirrorservice.org/sites/www.bitsavers.org/pdf/xer... If you want to try this stuff out for yourself (and don't have an Alto handy), use the ContrAlto emulator: https://github.com/livingcomputermuseum/ContrAlto https://github.com/livingcomputermuseum/ContrAlto

jecel 9y ago

An option is to use the BITBLT instruction which could be called from BCPL: ftp://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/BitBLT_Nov1975.pdf Dan Ingalls showed how to implement the Game of Life and how to rotate bitmaps using this instruction in a SIMD style. While BITBLT implemented all 16 possible boolean functions, not all were equally fast.